A

Access
Information about how and when a trail, park, or outdoor area can be reached or used. Access may be affected by road conditions, seasonal closures, permits, parking, accessibility, or other restrictions.

Acclimatization
The gradual process of adjusting to a different environment, especially higher altitude. At elevation, the body needs time to adapt to lower oxygen availability.

Altitude
Height above sea level. Higher altitudes have lower atmospheric pressure and less available oxygen, which can affect breathing, exertion, hydration needs, and how someone feels during activity.

Ambient Light
The overall light naturally or artificially present in an environment. Outdoors, ambient light includes light coming from the sky and surroundings rather than requiring you to look directly toward the sun.

Aspect
The direction a slope, surface, or feature faces, such as north, south, east, or west. Aspect can influence how much sunlight, wind, moisture, and heat a location receives, which can create different environmental conditions even within a relatively small area.

Association
A relationship observed between two factors in research. An association means they occur together in some pattern, but it does not by itself show that one caused the other.

Attention Restoration Theory (ART)
A theory proposing that certain environments, particularly natural ones, may help restore the capacity for directed attention after sustained mental effort. ART is an influential explanation for some observed effects of nature exposure, but it remains a theory being studied rather than settled fact.

Avalanche Terrain
Terrain where avalanches can start, travel, or deposit snow. Avalanche risk depends on factors including slope, snowpack, weather, and surrounding terrain; traveling in avalanche terrain requires knowledge and appropriate preparation.

B

Biological Night
The period when the body’s circadian system is signaling that it is nighttime. Biological night is related to internal circadian timing and may not perfectly match the hours of darkness outside or the time someone chooses to sleep.

Biodiversity
The variety of living things within an area, including variation among species, ecosystems, and genetic life. A biodiverse environment contains many different forms of life rather than just one or a few.

Blaze / Trail Blaze
A painted mark, symbol, marker, or other sign used to help identify and follow a trail. Blaze colors and meanings vary by trail system, so check local trail information rather than assuming a color has a universal meaning.

Blue Space
An outdoor environment where water is a prominent feature, such as a lake, river, stream, coast, pond, or canal. Researchers sometimes use the term when studying relationships between environments and human health or well-being.

Body Clock / Internal Clock
An informal term for the body’s biological timing systems. These systems help organize daily rhythms in sleep, alertness, hormone release, body temperature, and other processes.

Bright Light Therapy
The intentional use of bright artificial light, usually at particular times of day, to influence circadian timing or treat certain conditions such as seasonal affective disorder. Because timing, intensity, and individual circumstances matter, therapeutic use is more specific than simply making a room brighter.

Bryophyte
A group of small, non-vascular land plants that includes mosses, liverworts, and hornworts. Bryophytes do not have the same vascular system for transporting water and nutrients found in plants such as trees and flowering plants.

C

Cairn
A deliberately constructed pile or stack of stones. Cairns may sometimes be used as route markers, particularly where a trail is difficult to see, but not every rock pile is an official marker. Avoid building new cairns or altering existing ones unless directed by the land manager.

Canopy
The upper layer formed by the leaves, needles, and branches of trees and other tall vegetation. A forest canopy affects the amount of sunlight, rain, wind, and heat reaching the environment below it.

Cause and Effect / Causation
A relationship in which one factor actually produces a change in another. Showing causation requires stronger evidence than simply finding that two things occur together.

Circadian Rhythm
A biological rhythm that follows an approximately 24-hour cycle. Circadian rhythms help regulate sleep and wakefulness, alertness, body temperature, hormone activity, and many other processes.

Circadian Timing
The timing or phase of the body’s circadian rhythms relative to the day and night. Light is one of the most important environmental signals helping the circadian system stay synchronized with the outside world.

Compass
A navigation tool that uses Earth’s magnetic field to indicate direction. A traditional compass can be used with a map to help determine orientation and travel direction without requiring cellular service or a battery.

Correlation
A statistical relationship in which two variables change together in some way. Correlation can reveal an important pattern, but correlation alone does not establish that one variable caused the other.

D

Dawn Simulation
The gradual increase of artificial light around waking time to imitate the natural transition from darkness to morning. Dawn-simulation devices are often built into alarm clocks and have been studied for their effects on waking, circadian timing, and seasonal mood.

Daylight Exposure
Contact with natural light during daytime. The amount and characteristics of daylight exposure vary with time of day, season, weather, location, surroundings, and whether someone is indoors or outdoors.

Dehydration
A state in which the body has lost more fluid than it has taken in. Dehydration can affect physical and cognitive function and may become serious, particularly during prolonged activity, heat, illness, or limited access to fluids.

Difficulty Rating
A trail system’s estimate of how challenging a trail or route may be. Ratings can consider factors such as distance, elevation gain, grade, terrain, technical features, and exposure, but difficulty scales are not standardized everywhere. Always check how the particular park, app, or trail system defines its ratings.

Directed Attention
The deliberate, effortful kind of attention used to concentrate on something while ignoring distractions. Directed attention is an important concept in Attention Restoration Theory.

E

Ecosystem
A community of living organisms and the nonliving environment around them interacting as a system. An ecosystem can include plants, animals, fungi, microorganisms, water, soil, air, climate, and other environmental features.

Electrolytes
Minerals that carry an electrical charge when dissolved in body fluids. Electrolytes such as sodium and potassium are involved in functions including fluid balance, nerve signaling, and muscle function.

Elevation
The height of a location above a reference point, usually sea level. On a trail listing, elevation may describe the height of the trailhead, destination, highest point, or another location along the route.

Elevation Gain
The total amount of uphill climbing accumulated along a route. A five-mile walk with substantial elevation gain can feel very different from a five-mile walk on mostly level ground, which is why distance alone doesn’t tell you how demanding a trail may be.

Emergency / Satellite Communication
Methods of communicating when ordinary cellular service is unavailable or unreliable. Satellite messengers, personal locator beacons, and some satellite-enabled phones can provide emergency communication in remote areas, but their capabilities differ and should be understood before relying on them.

Epiphyte / Epiphytic
An organism that grows on another plant for physical support without taking nutrients from it in the way a parasite does. Many mosses, lichens, ferns, and other organisms can grow epiphytically on trees.

Estimated Time
An approximation of how long a trail or activity may take. Estimates are guides rather than guarantees; pace, terrain, elevation, weather, stops, fitness, group needs, and trail conditions can all affect actual time.

Evidence
Information used to support or evaluate a claim. In research, evidence can come from many kinds of studies, and its strength depends on factors such as study design, measurement quality, consistency, sample size, and the question being asked.

Exposure — Trail/Weather Context
How open or unprotected a person is to environmental conditions or terrain hazards. On a trail, exposure can refer to places with significant drop-offs or little protection from a fall; it can also describe limited shelter from sun, wind, storms, or other weather. Check the context when the word appears.

F

Field Guide
A reference designed to help identify or learn about things encountered in the natural world, such as birds, plants, trees, fungi, rocks, insects, or animal tracks. Field guides may be books, cards, digital resources, or apps.

Flash Flood
A rapid rise of water that can occur with little warning, often after intense rainfall or when water moves quickly through steep terrain, washes, creeks, or narrow channels. Dangerous flooding can occur even when rain is falling somewhere upstream rather than directly overhead.

G

Grade / Trail Grade
The steepness of a trail or slope, usually expressed as a percentage. Grade describes how steep the climb is, while elevation gain describes how much total climbing occurs along the route.

Green Space
An area where vegetation or other natural features are prominent, such as parks, gardens, woodland, trails, or areas of urban greenery. Researchers often use the term when studying relationships between environments and health or well-being.

H

Habitat
The environment in which an organism lives and finds the conditions and resources it

I

Identification App
A digital tool designed to help identify plants, animals, fungi, insects, or other things found in nature, often using photographs, sounds, location, or descriptive features. Identification apps can be useful starting points, but their suggestions are not always correct and may need confirmation from another reliable source.

J

Junction / Trail Junction
A place where two or more trails or routes meet, cross, or separate. Trail junctions are important navigation points; check signs, markers, or your map before continuing if the correct route isn’t clear.

L

Leave No Trace
A set of principles for reducing human impact while spending time outdoors. The seven Leave No Trace principles address planning and preparation, durable surfaces, waste, leaving what you find, fire, wildlife, and consideration for other visitors.

Light Box / Light Therapy Lamp
A device designed to provide bright artificial light for therapeutic purposes. Light boxes are used in bright light therapy and differ from ordinary household lamps in the intensity and characteristics of the light they provide.

Light Exposure
The light reaching a person over a period of time. Its biological effects can depend on factors including brightness, timing, duration, spectrum, and previous light exposure.

Light Sensitivity / Photosensitivity
An increased sensitivity or reaction to light. Certain medical conditions and medications can increase photosensitivity, which may be relevant when considering unusually bright light exposure or therapeutic light use.

Lightning Exposure
The degree to which a location or activity leaves someone vulnerable to lightning. Open areas, exposed ridges, summits, and some other terrain can increase risk during thunderstorms, making weather awareness and timely route decisions important.

Loop
A route that eventually returns to its starting point without requiring you to retrace the entire route. A loop may travel in either direction, and the difficulty can differ depending on which way it is completed.

Lux
A unit used to measure illuminance—the amount of visible light reaching a surface. Lux measurements are commonly used when comparing indoor lighting, daylight, and light-therapy devices.

M

Map Legend / Map Key
The part of a map that explains the symbols, colors, lines, and markings used on it. Reading the legend can help you distinguish trails, roads, water, boundaries, facilities, elevation information, and other mapped features.

Melatonin
A hormone involved in signaling biological night and helping coordinate circadian timing. Melatonin levels typically rise in the evening under normal light-dark conditions and are strongly influenced by light exposure.

Microclimate
The climate conditions within a small, specific area that may differ from those nearby. Shade, vegetation, water, terrain, wind, and sunlight can create differences in temperature, moisture, and other conditions over surprisingly short distances.

Moss
A small, non-vascular plant belonging to the bryophyte group. Mosses commonly grow in places where local conditions provide suitable moisture, and they can grow on soil, rocks, bark, fallen wood, and many other surfaces.

N

Native Species
A species that occurs naturally in a particular region or ecosystem without having been introduced there by humans. What counts as native depends on the geographic area and historical context being discussed.

Natural Environment
An environment made up primarily of natural features and processes, such as forests, wetlands, rivers, mountains, coastlines, grasslands, or other ecosystems. In research, exactly what qualifies as a natural environment can vary depending on how a study defines it.

Naturalist
Someone who observes, studies, or develops knowledge about the natural world. A naturalist does not necessarily have formal scientific training; the term can also describe someone who learns about nature through careful observation and sustained interest.

Nature-Based Intervention
A planned activity or program that intentionally incorporates contact with nature with the goal of influencing health or well-being. In research, this is different from simply observing that people happen to spend time outdoors.

Nature Exposure
A research term describing contact with natural environments or natural elements. Nature exposure might involve visiting a park or forest, spending time near water, viewing greenery, or other forms of contact, and its definition can vary considerably among studies.

Navigation
The process of determining where you are, where you are going, and how to travel between locations. Outdoor navigation can involve maps, compasses, trail markers, landmarks, GPS devices, apps, or a combination of tools.

Non-Vascular Plant
A plant without the specialized vascular tissues that many plants use to transport water and nutrients throughout the organism. Mosses, liverworts, and hornworts are non-vascular plants and generally remain relatively small. needs, such as food, water, shelter, space, or suitable places to reproduce.

O

Observational Study
A study in which researchers observe people, exposures, behaviors, or outcomes without assigning the conditions being studied. Observational studies can reveal important patterns and associations, but additional evidence is generally needed to determine whether one factor actually causes another.

Offline Map
A digital map downloaded to a phone or other device before a trip so it can be viewed without cellular or internet service. Offline maps can be useful where reception is unreliable, but battery life and device failure make it wise not to depend on a phone without considering a backup navigation plan.

Out-and-Back
A route that follows a path to a destination or turnaround point and then returns along the same route. The total listed distance generally includes both the trip out and the return.

P

Pack It In, Pack It Out
A principle of outdoor stewardship meaning that what you bring into an outdoor space should leave with you, including food wrappers, tissues, pet waste, and other trash. The goal is to avoid leaving waste behind for wildlife, other visitors, or land managers to deal with.

Phytoncides
A broad term commonly used for biologically active compounds produced by plants, particularly in discussions of forest environments and human health research. Some are volatile compounds released into the surrounding air. Research is investigating whether and how exposure to these compounds may contribute to observed effects associated with forest environments; claims about specific human-health benefits should be interpreted with appropriate caution.

Physiological Measure
A measurement of the body’s functioning rather than solely a person’s description of how they feel. Depending on the research question, examples might include heart rate, blood pressure, hormone levels, or other biological measurements.

Point-to-Point
A route that begins in one location and ends somewhere else rather than returning to the starting point. Point-to-point routes may require arranging transportation between the two ends.

R

Retina
The light-sensitive layer of tissue at the back of the eye. Cells in the retina detect light and send information to the brain for vision as well as biological processes including circadian timing.

Route
The path or course followed to travel from one place to another. A route may follow an established trail, combine several trails, or sometimes travel through areas without a clearly defined trail.

Route Type
The overall pattern a route follows. Common route types include loop, out-and-back, and point-to-point, and knowing the route type helps you understand how you’ll return or where you’ll finish.

S

Seasonal Access
Access to a trail, road, park, or outdoor area that changes during different times of year. Snow, wildfire risk, flooding, wildlife protection, maintenance, road closures, or other conditions may make an area inaccessible or restricted during certain seasons.

Seasonal Affective Disorder (SAD)
A type of depression characterized by a recurring seasonal pattern, most commonly beginning in fall or winter and improving during spring or summer, although other seasonal patterns can occur. Light therapy is one treatment used for some people with winter-pattern SAD.

Self-Reported Measure
Information provided directly by study participants about their own experiences, behaviors, symptoms, feelings, or perceptions. Self-reported measures can provide valuable information, but they measure something different from physiological or other objectively recorded measures.

Situational Awareness
Awareness of what is happening around you and how changing conditions may affect your safety or decisions. Outdoors, this can include noticing weather, terrain, daylight, trail conditions, wildlife, other people, your location, and how you or members of your group are doing.

Sleep Pressure
The biological drive for sleep that generally builds the longer a person remains awake and decreases during sleep. Sleep pressure works alongside the circadian timing system; they are related but distinct processes involved in regulating sleep and wakefulness.

Sleep-Wake Cycle
The recurring pattern of sleeping and waking across the day and night. Sleep-wake timing is influenced by both the body’s circadian system and the accumulation and release of sleep pressure.

Species
A basic category used by biologists to classify living organisms. In simple terms, a species groups organisms that share important biological characteristics, although the precise scientific definition can vary depending on the organisms and context.

Spectrum / Light Spectrum
The range of wavelengths contained in light. Different light sources have different spectral compositions, and the body’s visual and non-visual responses to light can vary depending partly on which wavelengths are present.

Suprachiasmatic Nucleus (SCN)
A small region of the brain that serves as the body’s primary circadian pacemaker, helping coordinate daily biological rhythms throughout the body. The SCN receives information about environmental light from the eyes, helping synchronize internal timing with the external day-night cycle.

Surface / Trail Surface
The material or ground underfoot on a trail, such as pavement, gravel, packed dirt, sand, rock, roots, boardwalk, or mud. Trail surface can affect footing, accessibility, speed, and how difficult a route feels.

Systematic Review
A structured research method used to identify, evaluate, and synthesize the available studies addressing a particular question. A systematic review can provide a broader view of the evidence than a single study, although its conclusions still depend on the quality and relevance of the research it includes.

T

Ten Essentials / 10 Essentials
A widely used framework for basic outdoor preparedness. The traditional categories cover navigation, illumination, sun protection, first aid, tools or repair supplies, fire, emergency shelter, extra food, extra water, and extra clothing. What you actually carry should be appropriate for the activity, environment, conditions, and length of your outing.

Terrain
The physical characteristics of the land you are traveling through. Terrain can include features such as hills, rocks, roots, slopes, water, loose ground, snow, or uneven surfaces and can significantly affect the difficulty of a route.

Trail
An established path or route intended for travel through an outdoor area. Trails may be designed for walking, hiking, cycling, horseback riding, multiple uses, or other activities, and their surfaces and levels of development vary considerably.

Trail Conditions
The current state of a trail and the factors that may affect travel on it. Conditions can include mud, snow, ice, flooding, fallen trees, erosion, construction, closures, wildfire effects, or other temporary circumstances. Recent trail reports can be useful because conditions may change much faster than permanent trail descriptions.

Trail Etiquette
Shared practices intended to help people use trails safely and respectfully. Etiquette can include yielding appropriately, managing pets, respecting other visitors, staying on designated routes where required, minimizing noise, and following rules specific to the land or trail system.

Trail Listing
A description of a trail or route provided by a park, land manager, guidebook, website, or app. Listings commonly include information such as distance, elevation gain, difficulty, route type, trailhead location, surface, access, and current conditions.

Trail Marker
A sign, symbol, post, blaze, emblem, or other marker used to identify a trail or help travelers follow its route. Marker styles and meanings vary among parks and trail systems.

Trail Stewardship
The care and responsible use of trails and the environments surrounding them. Stewardship can include minimizing personal impact, respecting closures and regulations, maintaining trails, volunteering, reporting problems, and supporting the long-term protection of outdoor spaces.

Trail System
A connected network of trails within a park, forest, community, recreation area, or other landscape. Because several routes may share sections or intersect within a trail system, understanding junctions and trail names can be important for navigation.

Trailhead
The designated starting or access point for a trail or trail system. A trailhead may include parking, signs, maps, restrooms, permits, information boards, or other facilities, but some are little more than a marked place where the trail begins.

Turnaround Time
A time chosen in advance to stop traveling farther and begin returning, regardless of whether the intended destination has been reached. Turnaround times can help protect against running out of daylight, exceeding available time, or continuing farther than conditions safely allow.

U

V

Variable
A factor or characteristic that can differ or change and can be measured or considered in research. Examples might include age, amount of nature exposure, sleep duration, stress ratings, or environmental conditions.

Volatile Organic Compounds (VOCs)
Carbon-containing compounds that readily become gases or vapors under ordinary environmental conditions. VOCs come from many natural and human-made sources; plants naturally release numerous VOCs, including compounds studied in research on forest environments.

W

Water Crossing
A place where a route passes through or across water, such as a creek, stream, or river. The difficulty and safety of a crossing can change substantially with water depth, current, weather, snowmelt, and recent rainfall, so conditions observed in an older trail report may no longer apply.

Water Filter
A device that physically removes organisms, sediment, or other material from water by passing it through a filtering medium. Filters differ in what they remove, and filtration is not the same as purification, so the device’s specifications and local water risks matter.

Water Purifier
A treatment method or device intended to make water safer to drink by addressing a broader range of disease-causing organisms than many basic filters. Purification methods vary, and no single method should be assumed to remove every possible contaminant.

Waypoint
A specific location recorded or marked for navigation, usually with geographic coordinates. A waypoint might identify a trailhead, junction, campsite, destination, water source, or another location someone may want to find or return to.

Wildfire Smoke
Air pollution produced by wildfires and composed of gases and fine particles. Smoke can travel far beyond the fire itself, so air quality and smoke conditions may affect an outdoor plan even when no wildfire is nearby.

X

Y

Z