Plain English Meaning
An FMS that takes information from several different navigation systems at once and combines them to figure out where the aircraft is and where it's going. If one source drops out, the others keep it working.
Full Definition
A Flight Management System that uses inputs from multiple navigation sources — typically GPS, DME, VOR, and inertial reference — and blends them to compute the aircraft's position, route, and guidance. Because it draws on more than one sensor, it can cross-check sources and continue providing accurate navigation if one source becomes unavailable or unreliable.