Plain English Meaning
A needle that tells you whether you are left or right of the course line you have chosen, and roughly how far off you are. Center the needle and you are on course.
Full Definition
An instrument display, usually a vertical needle on a navigation indicator, that shows how far left or right of the selected course the aircraft is. The needle centers when the aircraft is on course and deflects toward the side of the selected course the aircraft needs to fly to in order to return to it.
Context Anchor
Seen on navigation instruments and glass-panel displays when flying a selected course, especially during instrument flying.