Altitude is a term used to describe the height of an object above a reference point. It is often used to describe the height of an aircraft, a mountain, or a satellite above the Earth’s surface.
In aviation, altitude refers to the height of an aircraft above mean sea level (MSL), which is the average height of the ocean surface. Air traffic control uses this measurement to monitor the position of aircraft and to ensure safe separation between aircraft in flight.
In mountaineering, altitude refers to the height of a mountain peak above sea level. This measurement is used to rank mountains and to determine their relative heights.