Plain English Meaning

The invisible line down the center of a flight route that the aircraft is supposed to follow. Think of it like the painted center stripe on a road, except it's in the sky and you stay on it using navigation equipment instead of your eyes.

Full Definition

The imaginary line running down the exact middle of a published flight route, defined by a sequence of waypoints or navigation aids. Aircraft are expected to track this line within specified lateral tolerances when flying the route.

Context Anchor

Seen on RNAV departure and instrument procedure discussions when describing the exact path the aircraft should track after takeoff.

Why Pilots Care

Staying on the route centerline keeps the aircraft inside protected airspace, of and other . -- even if still legally within -- reduces the the procedure was designed to provide.