The Collaborative International Dictionary of English v.0.48:

Respirator \Res"pi*ra`tor\ (r?s"p?*r?`t?r), n. [Cf. F.
   respirateur.]
   A divice of gauze or wire, covering the mouth or nose, to
   prevent the inhalation of noxious substances, as dust or
   smoke. Being warmed by the breath, it tempers cold air
   passing through it, and may also be used for the inhalation
   of medicated vapors.
   [1913 Webster]

WordNet (r) 3.0 (2006):

respirator
    n 1: a breathing device for administering long-term artificial
         respiration [syn: respirator, inhalator]
    2: a protective mask with a filter; protects the face and lungs
       against poisonous gases [syn: gasmask, respirator, gas
       helmet]

The Devil's Dictionary (1881-1906):

RESPIRATOR, n.  An apparatus fitted over the nose and mouth of an
inhabitant of London, whereby to filter the visible universe in its
passage to the lungs.