Altitude sickness, or acute mountain sickness (AMS), is illness caused by ascending to high altitude faster than the body can acclimatize to the reduced oxygen. Symptoms include headache, nausea, fatigue, and dizziness. It can progress to the life-threatening conditions HAPE and HACE, so the response is to stop ascending and, if it worsens, descend.
Mountaineering is the sport of climbing mountains, combining hiking, rock, snow, and ice climbing with the skills to travel safely in high, remote, and glaciated terrain. It ranges from non-technical walk-up peaks to committing alpine ascents, and demands fitness, navigation, weather judgment, and hazard management alongside climbing ability.
A zero day, or 'zero', is a rest day on a long-distance hike when you cover zero trail miles — usually spent in a trail town resting, resupplying, eating, and recovering. A 'nero' (near-zero) is a very low-mileage day. Zeros are essential for recovery and morale on a thru-hike.
A trail angel is a person who voluntarily helps long-distance hikers — offering rides, food, a place to shower or sleep, or leaving caches of supplies. Trail angels are the people behind 'trail magic', and their generosity is a defining, much-loved feature of the thru-hiking community.
Trail magic is unexpected kindness shown to long-distance hikers — free food, drinks, rides, or help — often left at trailheads or given by strangers. A gift of trail magic can lift a weary thru-hiker enormously, and the people who provide it are known as trail angels. It is a beloved part of trail culture.
A trail name is a nickname a long-distance hiker adopts or is given on the trail, used in place of their real name within the hiking community. Trail names often arise from a memorable trait, mishap, or piece of gear, and are a cherished tradition of thru-hiking culture on trails like the Appalachian Trail.
A cat hole is a small hole, six to eight inches deep, dug to bury human waste in the backcountry where no toilet exists. It is the standard Leave No Trace method for disposing of solid waste, sited at least 200 feet from water, trails, and camp. Toilet paper is packed out or buried depending on the area.
A spur trail is a short side trail branching off a main trail, usually leading to a specific feature — a viewpoint, water source, campsite, or summit — and then dead-ending. Hikers take a spur out and back to reach the feature, then return to the main trail to continue.
A blowdown is a tree or large branch that has fallen across a trail, usually from wind, storms, or disease. Blowdowns force hikers to climb over, crawl under, or detour around them, and a trail thick with blowdown after a storm can dramatically slow progress until crews clear it.
Treeline (or timberline) is the elevation above which trees can no longer grow, due to cold, wind, and a short growing season. Above treeline the landscape becomes open alpine terrain — more exposed to weather, harder to navigate, and ecologically fragile — so hikers take extra care there.