an air or gas vent, especially one to carry off fumes from a tunnel, underground passage, etc.
either of two nostrils or spiracles, or a single one, at the top of the head in whales and other cetaceans, through which they breathe.
a hole in the ice to which whales or seals come to breathe.
Metallurgy. a defect in a casting or ingot caused by the escape of gas.
Geology. a hole in a sea cliff or coastal terrace through which columns of spray are jetted upward.

1685–95; blow2 + hole Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
blowhole (ˈbləʊˌhəʊl)
1.  the nostril, paired or single, of whales, situated far back on the skull
2.  a hole in ice through which whales, seals, etc, breathe
3.  a.  a vent for air or gas, esp to release fumes from a tunnel, passage, etc
 b.  (NZ) a hole emitting gas or steam in a volcanic region
4.  a bubble-like defect in an ingot resulting from gas being trapped during solidification
5.  geology a hole in a cliff top leading to a sea cave through which air is forced by the action of the sea

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

1787, of whales and porpoises, from blow (v.1) + hole.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
Unlike today's whales, it had no blowhole-the ancient behemoth had to raise its head above water to breathe.
The largest animal ever to have lived, the blue whale is a marvel of bone and
  blubber, blowhole and baleen-in immense proportions.
When the whale surfaces, the muscles open the blowhole and stale air comes out
  from the lungs.
However, there could have been one or more than one blowhole through the zinc
  chromate putty before ignition.
Image for blowhole
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