Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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}]