Plain English Meaning
The imaginary line that marks the exact track the aircraft is supposed to fly. Performance limits — how far the aircraft is allowed to drift — are measured outward from this line on either side.
Full Definition
The reference line down the middle of an RNP (Required Navigation Performance) flight path that defines the intended lateral track the aircraft is meant to follow. RNP containment requirements are measured as a distance either side of this centerline, expressed in nautical miles.
Context Anchor
Seen in Required Navigation Performance procedures, especially when describing how closely the aircraft must stay on the intended path during an approach or route segment.