The Collaborative International Dictionary of English v.0.48:

Air sac \Air" sac`\
   1. (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.
      [1913 Webster]

   2. An alveolus.
      [PJC]

WordNet (r) 3.0 (2006):

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 sac, 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