The Collaborative International Dictionary of English v.0.48:

Ventilator \Ven"ti*la`tor\, n. [Cf. F. ventilateur, L.
   ventilator a winnower.]
   A contrivance for effecting ventilation; especially, a
   contrivance or machine for drawing off or expelling foul or
   stagnant air from any place or apartment, or for introducing
   that which is fresh and pure.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ventilator
    n 1: a device (such as a fan) that introduces fresh air or
         expels foul air
    2: a device that facilitates breathing in cases of respiratory
       failure [syn: breathing device, breathing apparatus,
       breathing machine, ventilator]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "ventilator":
   aerator, air conditioner, air cooler, air duct, air filter,
   air hole, air passage, air shaft, air tube, airway, blower,
   blowhole, breathing hole, chiller, cold pack, cooler,
   cooling system, electric fan, exhaust fan, fan, flabellum, ice bag,
   ice bucket, ice cube, ice pack, ice pail, louver, louverwork,
   naris, nostril, punkah, shaft, spilehole, spiracle, surface cooler,
   thermantidote, touchhole, transom, vent, ventage, venthole,
   ventiduct, ventilating shaft, water cooler, wind tunnel,
   windcatcher, windsail, windscoop, wine cooler