The Collaborative International Dictionary of English v.0.48:

airgun \air"gun`\, air gun \air" gun`\(g[u^]n`).
   A kind of gun in which the elastic force of condensed air is
   used to discharge the ball. The air is powerfully compressed
   into a reservoir attached to the gun, by a condensing pump,
   and is controlled by a valve actuated by the trigger. The
   common BB gun is a type of air gun.
   [1913 Webster]

WordNet (r) 3.0 (2006):

air gun
    n 1: a gun that propels a projectile by compressed air [syn:
         air gun, airgun, air rifle]