The term Above ground level is used in aviation, atmospheric sciences and broadcasting, and is the height above ground level (AGL) measured with respect to the underlying ground surface.
This is as opposed to altitude/elevation above mean sea level (AMSL), or (in broadcast engineering) height above average terrain (HAAT). In other words, these expressions (AGL, AMSL, HAAT) indicate where the "zero level" or "reference altitude" is located.