Plain English Meaning
Specific compass headings that ATC tells a pilot to fly. The controller is watching the aircraft on a radar screen and telling the pilot exactly which direction to point the airplane to get it where it needs to go.
Full Definition
Headings issued by an air traffic controller to a pilot, based on the controller's observation of the aircraft's position on radar, used to guide the aircraft along a specific ground track for traffic separation, sequencing, navigation assistance, or to position the aircraft for an approach.