Plain English Meaning
The straight lines down the middle of a runway, taxiway, or approach path that pilots line up with to stay centered.
Full Definition
The imaginary lines running lengthwise down the exact middle of runways, taxiways, or final approach courses, used as the primary lateral reference for alignment during taxi, takeoff, approach, and landing. On runways and taxiways, they are marked with painted stripes; for instrument approaches, they are defined electronically by the navigation aid serving that runway.
Context Anchor
Seen in airport diagrams, , and discussions about closely spaced .