Definition
Authorizations issued by an air traffic control facility for an aircraft to proceed under specified traffic conditions within controlled airspace. A clearance defines the route, altitude, and any restrictions or instructions the pilot must follow, and is required before operating IFR in controlled airspace.
Plain English
An ATC clearance is permission from air traffic control to fly a specific route and altitude under specific conditions. The pilot must read it back, understand it, and follow it.
Context Anchor
You may encounter ATC clearances on the radio from a controller, through Flight Service when direct contact with air traffic control is not available, or in written instructions before an instrument flight.
Derivation
Clearance comes from the Latin clarus, meaning clear or bright, and later took the sense of 'making the way open.' An ATC clearance is exactly that — ATC clearing a path through traffic for the aircraft to use.
Why Pilots Care
They ensure safe separation from other traffic and keep the flight legal within the airspace rules.
Intuition Check
Do not read clearance as general approval to do whatever seems next. An ATC clearance is specific permission with specific limits.
Example Sentence 1
Before taxiing for an IFR departure, the pilot called clearance delivery and copied the ATC clearance, then read it back.
Example Sentence 2
Before entering Class B airspace the crew requested and received the required ATC clearances.