From WordNet (r) 3.0 (2006) [wn]:
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]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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.
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Note: There are two spiracles or blowholes in the common
whales, but only one in sperm whales, porpoises, etc.
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3. A hole in the ice to which whales, seals, etc., come to
breathe.
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4. (Founding) An air hole in a casting.
[1913 Webster]