Plain English Meaning
How high the aircraft is above the ground right below it. If the ground rises, your AGL height drops, even if you haven't climbed or descended.
Full Definition
An altitude measured vertically from the surface of the ground directly beneath the aircraft, rather than from sea level. AGL values change as the terrain below the aircraft rises or falls, even when the aircraft maintains a constant altitude above sea level.
Context Anchor
Seen in takeoff, landing, traffic pattern, obstacle clearance, and low-altitude flying discussions.
Why Pilots Care
AGL tells a pilot the true height above and ground, which is essential for safe , avoidance, and with that are stated in AGL.