Definition

Airworthiness Directive (AD). A regulatory document which identifies aeronautical products in which an unsafe condition exists, and where the unsafe condition is likely to exist or develop in other aeronautical products of the same type design. It prescribes mandatory corrective actions to be taken, or the conditions or limitations under which the aeronautical products may continue to be operated. The AD is the  common form of mandatory continuing airworthiness information mentioned in Annex 8.

Source: ICAO Doc 9760 - Airworthiness Manual