Plain English Meaning
The pointer on the VOR instrument that tells you whether you are on the line you've chosen to fly, or off to the left or right of it. Center means on course; needle leaning left or right means the course is over there.
Full Definition
The course deviation indicator (CDI) on a VOR display — a vertical needle that swings left or right of center to show whether the aircraft is left or right of the selected VOR course. When the needle is centered, the aircraft is on the chosen course; full-scale deflection means the aircraft is at or beyond the edge of the course width (typically 10° either side of the selected radial).
Context Anchor
Seen on VOR displays and in discussions comparing VOR indicators with needles.