Plain English Meaning
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.
Full 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.
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 on the .