The Collaborative International Dictionary of English v.0.48:

Blowhole \Blow"hole`\, n.
   1. A cavern in a cliff, at the water level, opening to the
      air at its farther extremity, so that the waters rush in
      with each surge and rise in a lofty jet from the
      extremity.
      [1913 Webster]

   2. A nostril or spiracle in the top of the head of a whale or
      other cetacean.
      [1913 Webster]

   Note: There are two spiracles or blowholes in the common
         whales, but only one in sperm whales, porpoises, etc.
         [1913 Webster]

   3. A hole in the ice to which whales, seals, etc., come to
      breathe.
      [1913 Webster]

   4. (Founding) An air hole in a casting.
      [1913 Webster]

WordNet (r) 3.0 (2006):

blowhole
    n 1: the spiracle of a cetacean located far back on the skull
    2: a hole for the escape of gas or air [syn: vent, venthole,
       vent-hole, blowhole]

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "blowhole":
   air duct, air hole, air passage, air shaft, air tube, airway,
   armhole, avenue, 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, spiracle, spout, tap,
   touchhole, transom, vent, ventage, venthole, ventiduct,
   ventilating shaft, ventilator, vomitory, way out, weir,
   wind tunnel