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]