Plain English Meaning
A straight, invisible line drawn from your eye to something you're looking at. In flying, you use this line as a reference — for example, keeping the wing pointed exactly at a spot on the ground.
Full Definition
An imaginary straight line extending from the pilot's eye to a specific reference point, used as a visual cue to maintain orientation, position, or alignment relative to that point. In eights-on-pylons, the lateral axis of the airplane is held on this line so that an extension of the wing appears to pivot on the chosen pylon.
Context Anchor
In eights on pylons, the pilot uses the line of sight to judge whether the selected ground point appears to in the correct position beside the airplane.