Plain English Meaning
An onboard computer that holds the flight plan, tracks where the aircraft is, and tells the autopilot where to go next. The pilot loads the route into it before takeoff, and it manages navigation through the whole flight.
Full Definition
A computer system on the aircraft that combines navigation, flight planning, and performance data into a single integrated unit. The FMS uses inputs from sensors such as GPS, inertial reference systems, and radio navigation aids to determine the aircraft's position, then guides the aircraft along a programmed route by sending to the autopilot and .