Source : Webster's Revised Unabridged Dictionary (1913)

Exhaust \Ex*haust"\, a. [L. exhaustus, p. p.]
   1. Drained; exhausted; having expended or lost its energy.

   2. Pertaining to steam, air, gas, etc., that is released from
      the cylinder of an engine after having preformed its work.

   {Exhaust draught}, a forced draught produced by drawing air
      through a place, as through a furnace, instead of blowing
      it through.

   {Exhaust fan}, a fan blower so arranged as to produce an
      exhaust draught, or to draw air or gas out of a place, as
      out of a room in ventilating it.

   {Exhaust nozzle}, {Exhaust orifice} (Steam Engine), the blast
      orifice or nozzle.

   {Exhaust pipe} (Steam Engine), the pipe that conveys exhaust
      steam from the cylinder to the atmosphere or to the
      condenser.

   {Exhaust port} (Steam Engine), the opening, in the cylinder
      or valve, by which the exhaust steam escapes.

   {Exhaust purifier} (Milling), a machine for sorting grains,
      or purifying middlings by an exhaust draught. --Knight.

   {Exhaust steam} (Steam Engine), steam which is allowed to
      escape from the cylinder after having been employed to
      produce motion of the piston.

   {Exhaust valve} (Steam Engine), a valve that lets exhaust
      steam escape out of a cylinder.

Source : WordNet®

exhaust fan
     n : a fan that moves air out of an enclosure