1. The measurement of height, usually given in meters above sea level: The guide told the tourists to take along warm clothes because as the altitude increases, the colder it gets when hiking up the mountains in the Alps.
2. The angular distance: Altitude is usually measured in degrees, above the horizon, from zero degrees at the horizon to 90 degrees at the zenith.

One of the two co-ordinates (the other being azimuth) that define a celestial object's position, used with an altazimuth mount.