(1):

(n.) A nostril or spiracle in the top of the head of a whale or other cetacean.

(2):

(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.

(3):

(n.) An air hole in a casting.

(4):

(n.) A hole in the ice to which whales, seals, etc., come to breathe.