1. A measure (or condition) of height, especially of great height, as a mountain top or aircraft flight level. In meteorology, altitude is used almost exclusively with respect to the height of an airborne object above the earth's surface, above a constant-pressure surface, or above mean sea level. The measurement of altitude is accomplished by altimeters in aeronautics, and the entire study is called altimetry. 2. In astronomy, same as elevation angle.