Plain English Meaning
A radio beacon on the ground that lets a pilot know which direction the aircraft is from the station, so it can be used to fly along a chosen line in or out of that station.
Full Definition
A ground-based radio navigation system that transmits a signal in all directions, allowing a properly equipped aircraft to determine its bearing to or from the station along any of 360 selectable courses (radials). The system operates in the very high frequency (VHF) band between 108.0 and 117.95 MHz and provides the basis for much of the airway and instrument approach structure in the United States.