The Collaborative International Dictionary of English v.0.48:

anti-aircraft \an`ti-air"craft\, a.
   designed for or used for defense against attack by aircraft;
   as, anti-aircraft cannon.
   [PJC]

WordNet (r) 3.0 (2006):

antiaircraft
    adj 1: designed for defense from a surface position against air
           attack
    n 1: artillery designed to shoot upward at airplanes [syn:
         antiaircraft, antiaircraft gun, flak, flack, pom-
         pom, ack-ack, ack-ack gun]