Definition
An authorization issued by an air traffic control facility for an aircraft to proceed under specified traffic conditions within controlled airspace. The clearance specifies the route, altitude, and any restrictions the pilot must follow, and it is issued primarily to prevent collisions between known aircraft.
Plain English
Permission from air traffic control telling the pilot exactly how, where, and at what altitude to fly through controlled airspace. Once the pilot accepts it, they are expected to follow it.
Context Anchor
Pilots encounter ATC clearances on the radio before takeoff, before entering certain controlled airspace, and while being guided during instrument flight.
Derivation
Clearance comes from the older sense of 'clearing the way' — making a path open and free of obstruction. An ATC clearance does the same thing for an aircraft: it clears a specific path through the sky for that flight.
Why Pilots Care
Required to legally operate in controlled airspace and central to collision avoidance between aircraft.
Intuition Check
Do not read “clearance” as a guarantee that the sky is empty or that the pilot may ignore other rules. It means ATC has authorized a specific action under specific conditions.
Example Sentence 1
Before taxiing for an IFR departure, the pilot called Clearance Delivery and copied the ATC clearance to their destination.
Example Sentence 2
Without an air traffic control clearance the aircraft remained outside Class B airspace as required.