Plain English Meaning
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.
Full 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.
Context Anchor
You may encounter ATC clearances on the radio from a controller, through Flight Service when with air traffic control is not , or in written instructions before an .