• WordNet 3.6
    • n blowhole a hole for the escape of gas or air
    • n blowhole the spiracle of a cetacean located far back on the skull
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Webster's Revised Unabridged Dictionary
    • Blowhole 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.
    • Blowhole A hole in the ice to which whales, seals, etc., come to breathe.
    • Blowhole A nostril or spiracle in the top of the head of a whale or other cetacean.
    • Blowhole (Founding) An air hole in a casting.
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Century Dictionary and Cyclopedia
    • n blowhole The nostril of a cetacean, generally situated on the highest part of the head. In the whalebone whales the blow-holes form two longitudinal slits, placed side by side. In porpoises, grampuses, etc., they are reduced to a single crescent-shaped opening.
    • n blowhole A hole in the ice to which whales and seals come to breathe.
    • n blowhole Same as air-hole, 2.
    • n blowhole In steel-manuf., a defect in the iron or steel, caused by the escape of air or gas while solidification was taking place.
    • n blowhole An orifice on a rocky coast connected with a passage which gives access to the waves. When a heavy roller dashes into the latter it spouts from the blow-hole.
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In literature:

The steam from the volcanic blowholes swayed below us.
"The Mystery" by Stewart Edward White and Samuel Hopkins Adams
It is different from the sperm whale, which has only one blowhole, and that a little one.
"Fighting the Whales" by R. M. Ballantyne
The greatest source of trouble is want of homogeneity in the casting, as evidenced by blowholes, etc.
"On Laboratory Arts" by Richard Threlfall
Head flat in front of blowhole, viewed from side.
"Whales, Dolphins, and Porpoises of the Western North Atlantic" by Stephen Leatherwood
Or the carpenter could put a coat of varnish over it and stop all the blowholes.
"The Dealings of Captain Sharkey" by A. Conan Doyle
A short frenzied search revealed a blowhole in the snow.
"My Attainment of the Pole" by Frederick A. Cook
They are known as "piping," "blowholes" and "segregation.
"Encyclopaedia Britannica, 11th Edition, Volume 14, Slice 7" by Various

In news:

Say hello to the Kraken blowholes that are Montreal's manholes.
Helicopters collect whale snot from blowholes.
A small, remote-controlled helicopter maneuvers just above the blowhole mist of a blue whale in the Gulf of California.
The doodle features Captain Ahab and his crew alongside the famous whale, who sits underneath the Google logo and replaces the "L" with water from his blowhole.