Plain English Meaning
The people on the ground (usually heard on the radio) who tell pilots what to do so airplanes stay safely apart and move smoothly through busy airspace and airports.
Full Definition
Federally certified personnel working in control towers, approach/departure facilities, and en route centers who issue instructions and clearances to pilots in order to maintain safe separation between aircraft and an orderly flow of traffic in controlled airspace and on airport surfaces.
Context Anchor
Pilots encounter air traffic controllers during radio communication for taxi, , , , airspace , and assistance.