Source : Webster's Revised Unabridged Dictionary (1913)

Air sac \Air" sac`\ (Anat.)
   One of the spaces in different parts of the bodies of birds,
   which are filled with air and connected with the air passages
   of the lungs; an air cell.

Source : WordNet®

air sac
     n 1: a tiny sac for holding air in the lungs; formed by the
          terminal dilation of tiny air passageways [syn: {alveolus},
           {air cell}]
     2: any of the thin-walled extensions of the tracheae of insects
     3: any of the membranous air-filled extensions of the lungs of
        birds