Plain English Meaning
The middle of the path you are supposed to follow. If you are right on the course centerline, you are not drifting left or right of where the course is taking you.
Full Definition
The imaginary line down the exact middle of a published navigation course, such as a localizer beam, VOR radial, or RNAV/GPS course. It represents zero lateral deviation — the precise track the aircraft is intended to fly along that course.
Context Anchor
Seen during instrument navigation and approach flying, especially when comparing the airplane’s position to the desired path shown by the instruments.