Plain English Meaning
An official FAA order that tells owners and operators they must fix a known safety problem on a specific type of aircraft or part. The aircraft is not legal to fly until the required action is done.
Full Definition
A legally enforceable rule issued by the FAA when an unsafe condition is found to exist in an aircraft, engine, propeller, or appliance, and that condition is likely to exist or develop in other items of the same type design. Compliance with applicable Airworthiness Directives is mandatory before the aircraft may be flown.
Context Anchor
You will see ADs during aircraft ownership, maintenance planning, preflight document checks, and discussions about whether an aircraft is legal and safe to fly.