Plain English Meaning
The instrument in the cockpit that tells the pilot how high the aircraft is above the ground right below it, not above sea level. It also lets the pilot set a height where it will warn them as they pass through it on approach.
Full Definition
A cockpit display that shows the aircraft's height directly above the terrain or surface beneath it, as measured by a radar altimeter. It typically presents the height in feet above ground level (AGL) and includes a pilot-set decision height (DH) bug that triggers a visual or aural alert when the aircraft descends through that height.