Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. A nostril or spiracle in the top of the head of a whale or
      other cetacean.

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

   3. A hole in the ice to which whales, seals, etc., come to
      breathe.

   4. (Founding) An air hole in a casting.

Source : WordNet®

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}]