Plain English Meaning
A radio station on the ground that sends out signals in all directions. An aircraft receiver can tune in and read which direction the station is from the aircraft, which lets the pilot fly toward it, away from it, or along a chosen line through it.
Full Definition
A ground-based radio navigation station that transmits signals in every direction, allowing a properly equipped aircraft to determine its bearing to or from the station along any of 360 selectable courses (radials). In civil aviation this almost always refers to the VHF Omnidirectional Range (VOR), which operates on frequencies between 108.0 and 117.95 MHz.
Context Anchor
Seen in navigation discussions, especially when how VOR shows the pilot a to or from a ground station.