From the Pilot/Controller Glossary
AIR TRAFFIC CONTROL SERVICE [ICAO]- A service provided for the purpose of:
a. Preventing collisions:
1. Between aircraft; and
2. On the maneuvering area between aircraft and obstructions.
b. Expediting and maintaining an orderly flow of air traffic.
Air traffic control (ATC) is is a service provided by ground-based controllers who direct aircraft on the ground and in the air. The primary purpose of ATC is to separate aircraft, to organize and expedite the flow of traffic, and to provide information and other support for pilots when able. In some countries, ATC may also play a security or defense role (as in the United States), or be run entirely by the military (as in Brazil).
ATC services in the United States are provided at three levels:
- Individual airport control towers ("Tower", "Ground", and "Clearance Delivery")
- Approach controllers ("Approach", "Departure")
- ARTCC Center controllers ("Center")
Per AIM 5-5-1:
- The roles and responsibilities of the pilot and controller for effective participation in the ATC system are contained in several documents. Pilot responsibilities are in the CFRs and the air traffic controllers' are in the FAA Order JO 7110.65, Air Traffic Control, and supplemental FAA directives. Additional and supplemental information for pilots can be found in the current Aeronautical Information Manual (AIM), Notices to Airmen, Advisory Circulars and aeronautical charts.