The Collaborative International Dictionary of English v.0.48:

Spiracle \Spir"a*cle\, n.[L. spiraculum, fr. spirare to breathe:
   cf. F. spiracule. See Spirit.]
   1. (Anat.) The nostril, or one of the nostrils, of whales,
      porpoises, and allied animals.
      [1913 Webster]

   2. (Zool.)
      (a) One of the external openings communicating with the
          air tubes or tracheae of insects, myriapods, and
          arachnids. They are variable in number, and are
          usually situated on the sides of the thorax and
          abdomen, a pair to a segment. These openings are
          usually elliptical, and capable of being closed. See
          Illust. under Coleoptera.
      (a) A tubular orifice communicating with the gill cavity
          of certain ganoid and all elasmobranch fishes. It is
          the modified first gill cleft.
          [1913 Webster]

   3. Any small aperture or vent for air or other fluid.
      [1913 Webster]

WordNet (r) 3.0 (2006):

spiracle
    n 1: a breathing orifice

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "spiracle":
   air duct, air hole, air passage, air shaft, air tube, airway,
   armhole, avenue, blowhole, breathing hole, bullet-hole, bunghole,
   channel, chute, cringle, deadeye, debouch, door, egress, emunctory,
   escape, estuary, exhaust, exit, eye, eyelet, floodgate, flume,
   gasket, grommet, guide, keyhole, knothole, loop, loophole, louver,
   louverwork, manhole, mousehole, naris, nostril, opening, out,
   outcome, outfall, outgate, outgo, outlet, peephole, pigeonhole,
   pinhole, placket, placket hole, pore, port, porthole, punch-hole,
   sally port, shaft, sluice, spilehole, spout, tap, touchhole,
   transom, vent, ventage, venthole, ventiduct, ventilating shaft,
   ventilator, vomitory, way out, weir, wind tunnel