The trachea.
A flight path used by aeroplanes.
The spiracle, on the top of the head, through which cetaceans breathe.
A top-facing opening to a cavity in the ground very near an ocean's shore, leading to a marine cave from which wave water or bursts of air are expelled.
A vertical opening in the top of a computer case that lets hot air (primarily from the CPU heat sink) escape quickly.
A vent for the escape of steam or other gas.
An unintended cavity filled with air in a casting product.
To fill or be filled with air in an unintended cavity.