Official Aviation Definition
Above Ground Level (AGL) refers to an altitude expressed in feet or meters measured directly above the terrain or water surface immediately beneath an aircraft. It differs from Mean Sea Level (MSL), which measures altitude relative to average sea level regardless of ground elevation.
In Simple Words
AGL is your exact height above whatever is directly underneath you—trees, buildings, or dirt.
Real-World Flight Example
When flying over a mountain plateau at 5,000 feet MSL where the ground is 4,000 feet high, the aircraft is flying at 1,000 feet AGL.