Plain English Meaning
How high the aircraft is, measured from a stated starting point — usually sea level or the ground beneath it.
Full Definition
Height expressed in units of distance above a reference plane, usually mean sea level (MSL) or ground level (AGL). In aviation, altitude is reported in feet and is qualified by the reference used: MSL altitude is measured from average sea level; AGL altitude is measured from the terrain directly below the aircraft; pressure altitude is the altitude indicated when the altimeter is set to 29.92 inches of mercury; density altitude is pressure altitude corrected for non-standard temperature.