Plain English Meaning
One instrument that shows you which way the aircraft is pointing and, at the same time, whether you are left or right of the route you have selected to fly. It puts heading and course information together so you can see your situation at a glance.
Full Definition
A flight instrument that combines the heading indicator and the VOR/ILS course deviation indicator into a single display. It shows the aircraft's heading, the selected course, the deviation of the aircraft from that course, and the to/from indication relative to a navigation station, all referenced to a compass card that with the aircraft's heading.