Plain English Meaning
A curved part of a flight path where the aircraft flies in a perfect arc at a set distance from a chosen point, ending at a specific waypoint. The curve is the same shape every time, so every aircraft flies it the same way.
Full Definition
A flight path segment defined by a fixed-radius curve around a specified center fix, used in RNAV procedures to guide the aircraft along a precise circular ground track between two waypoints. In ARINC 424 path and terminator coding, it is identified as the RF (Radius-to-Fix) leg type and is the only curved RNP leg flown with the same accuracy and repeatability as a straight segment.