|The angle of a celestial object measured upwards from the observer's horizon. Thus, an object on the horizon has an altitude of 0° and one directly overhead has an altitude of 90°. Negative values for the altitude mean that the object is below the horizon.
Altitude is usually used together with azimuth to give the direction of an object in the topocentric coordinate system.
Altitude is also frequently referred to as "elevation", and this term was used until recently by Heavens-Above. We now use the term "altitude" to be in line with most other astronomy publications, and reserve "elevation" to indicate the height of the observer above sea-level.