The description of the word you requested from the astronomical dictionary is given below.
Altitude and elevation measure how far something is above some reference plane. In daily life, they are used to indicate how far above sea level something is. In astronomy, they are used for that as well, but also to indicate how far (in degrees) something like a planet or a star is above the horizon. This can be confusing, so the preferred use is for elevation to be used for the height above sea level (in meters or feet), and altitude for height above the horizon (in degrees).
In the horizontal coordinate system, altitude is the coordinate that measures the height above the horizon (in degrees). The other coordinate is the azimuth. Because the true horizon depends on the local landscape and the exact location of the observer, astronomers often use an "artificial" horizon that runs exactly midway between the zenith and nadir. If you read about astronomical altitudes, then you may assume they are measured relative to the artificial horizon, unless the accompanying text says otherwise.