Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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