Plain English Meaning
How well the aircraft's navigation computer can figure out where the airplane is and keep it on the planned route within a stated margin of error.
Full Definition
A measure of how accurately an aircraft's flight management system (FMS) or area navigation (RNAV) computer can calculate and maintain the aircraft's position along a defined flight path. It reflects the navigation computer's ability to meet a specified accuracy standard, expressed as a lateral deviation value the system is expected to hold under normal conditions.