The Collaborative International Dictionary of English v.0.48:

Air pump \Air" pump`\
   1. (Physics) A kind of pump for exhausting air from a vessel
      or closed space; also, a pump to condense air or force it
      into a closed space.
      [1913 Webster]

   2. (Steam Engines) A pump used to exhaust from a condenser
      the condensed steam, the water used for condensing, and
      any commingled air.
      [1913 Webster]

WordNet (r) 3.0 (2006):

air pump
    n 1: a pump that moves air in or out of something [syn: air
         pump, vacuum pump]