A decision height (DH) or decision altitude (DA) is a specified height or altitude point/level used as a reference. This altitude specifically states which type of altitude a pilot is referring to. The term "altitude" is normally used to refer to the altitude above sea level, which is used to depict obstacles and airspace as well as separate air traffic. Pilots are mainly concerned with five types of altitude:
Indicated altitude is the altitude that is read directly from the altimeter when it is set to the current altimeter.
True altitude is the vertical distance from the aircraft above sea level; the actual altitude.
Absolute altitude is the vertical distance of an aircraft above the terrain or ground level.
Pressure altitude is the altitude indicated when the altimeter setting window is adjusted to 29.92.
Density altitude is the pressure altitude corrected for variations from standard temperature.