Plain English Meaning
A federally run service that pilots contact -- by phone before a flight or by radio in flight -- to get weather, file or open a flight plan, and receive other flight-related help. It is not air traffic control; it is a support service for pilots.
Full Definition
An air traffic facility operated by the FAA that provides pilots with weather briefings, flight plan filing and processing, en route communications, search and rescue services, assistance to lost or distressed aircraft, relay of air traffic control clearances, and broadcast of aviation weather and .