Altitude is the vertical angle between a given direction—such as the direction toward a particular point on the horizon from a given place—and the horizontal plane through the observer. A positive altitude indicates that the point being observed is above the observer; if it is below, then the altitude will be negative. Thus the altitude of a horizon point level with the observer is 0°. That of the summit of a high or nearby hill might be as much as 5° or 10°, but that of a sea horizon viewed from a high place might be -0.5° or -1°.
One can also speak of the altitude of a star in the sky, but this will not generally be the same as the angle of the star above the horizon, since the horizon altitude will not normally be 0°.
There is considerable confusion between the terms altitude and elevation. Elevation is normally taken to mean the height of a given place above sea level, but the two terms are quite often transposed, for example when pilots speak of the altitude of an airplane to mean its height above sea level and use an altimeter to measure it, while some engineers and astronomers use “elevation” to mean “altitude” as we have defined it here.