Plain English Meaning
A radio station on the ground that sends out a signal in every direction at once. A receiver in the aircraft points an arrow at the station, so the pilot knows which way it is.
Full Definition
A ground-based, low or medium frequency radio transmitter that radiates a signal equally in all directions. An aircraft equipped with an Automatic Direction Finder (ADF) receives the signal and displays the bearing from the aircraft to the station, allowing the pilot to track to or from the beacon.
Context Anchor
Seen on navigation charts, approach charts, and in radio navigation discussions, especially with older or backup .