blowhole

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An opening or one of a pair of openings for breathing, located on the top of the head of cetaceans, such as whales and dolphins. The blowhole is opened by muscles upon surfacing and closed by the pressure of water upon diving.
2. n. A hole in ice to which aquatic mammals, such as dolphins, come to breathe.
3. n. A vent to permit the escape of air or other gas.

Century Dictionary and Cyclopedia

1. n. 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.
2. n. A hole in the ice to which whales and seals come to breathe.
3. n. Same as air-hole, 2.
4. n. In steel-manuf., a defect in the iron or steel, caused by the escape of air or gas while solidification was taking place.
5. n. 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.

Wiktionary

1. n. The spiracle, on the top of the head, through which cetaceans breath.
2. n. A vent for the escape of gas.
3. n. A top-facing opening to a cavity in the ground very near an ocean's shore, that leads to a marine cave from which wave water and/or bursts of air are expelled.
4. n. An unintended cavity filled with air in a casting product.
5. n. A vertical opening in the top of computer cases, that let hot air, primarily from the CPU heat sink, escape quickly.

GNU Webster's 1913

1. n. 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. n. A nostril or spiracle in the top of the head of a whale or other cetacean.
3. n. A hole in the ice to which whales, seals, etc., come to breathe.
4. n. (Founding) An air hole in a casting.

WordNet 3.0

1. n. the spiracle of a cetacean located far back on the skull
2. n. a hole for the escape of gas or air