Definition
An authorization issued by air traffic control (ATC) that permits a pilot to operate an aircraft under instrument flight rules within controlled airspace, specifying the route, altitude, and any restrictions the pilot must follow.
Plain English
Permission from air traffic control to fly using instruments through controlled airspace, telling you where to go, how high to fly, and any rules to follow along the way.
Context Anchor
A pilot may receive an IFR clearance before departure, in the air before entering controlled airspace, or through a flight service station when direct radio contact with air traffic control is not available on the ground.
Derivation
IFR stands for Instrument Flight Rules. Clearance comes from the idea of being 'cleared' or given permission to proceed -- in this case, permission to fly under instrument rules.
Why Pilots Care
It provides separation from other traffic and enables safe flight through clouds or low visibility.
Intuition Check
Do not read “clearance” as a guarantee that everything ahead is safe or obstacle-free. Here it means official permission with instructions and limits that the pilot must follow.
Example Sentence 1
Before takeoff, the pilot called the Flight Service Station to receive an IFR clearance for the flight to Denver.
Example Sentence 2
After engine start the crew called clearance delivery to obtain their IFR clearance for the cross-country flight.