An altimeter is a device used to measure altitude, or the height above a specific reference point, usually sea level. It is commonly found in aircraft, where it helps pilots determine their elevation during flight. Altimeters can operate using different principles, including barometric pressure changes or GPS signals. In addition to aviation, altimeters are also used in various outdoor activities, such as hiking and mountaineering, to help adventurers track their elevation gain. Some modern smartphones even include altimeter features, making it easier for users to monitor their altitude while exploring nature or engaging in sports like mountain biking or rock climbing.