noun

1. a nostril in the top of the head of whales and certain other cetaceans, through which they breathe
2. a hole through which gas or air can escape, esp. in lava
3. a hole in the ice to which seals, whales, etc. come to get air
4. a vertical opening or chimney in the roof of a sea cave through which air and water are forced by the action of the waves and the rising tides
5. a flaw in cast metal caused by an air or gas bubble