Definition 1 of 2
Plain English Meaning
Orders from air traffic controllers telling a pilot what to do — where to go, how high to fly, when to turn, or when to take off or land. Pilots must follow them unless safety requires otherwise.
Full Definition
Directives issued by air traffic controllers requiring a pilot to take specific actions, such as a heading, altitude, route, speed, or clearance to taxi, take off, or land. Compliance is mandatory unless the pilot declares an emergency or determines that following the instruction would compromise safety, in which case the pilot must advise ATC.