Plain English Meaning
The help and instructions ATC gives pilots — keeping aircraft apart, passing on traffic and weather, issuing clearances, and assisting in emergencies. What's available depends on where you are flying and how busy the controller is.
Full Definition
The range of assistance, information, and control functions that Air Traffic Control provides to pilots, including separation of aircraft, traffic advisories, weather information, navigation assistance, clearances, and emergency support. The level of service available depends on the type of airspace, the controller's workload, , and whether the is operating under or .