1A chamber or vessel containing air; specifically (a) (in a pump or other hydraulic machine) a receptacle containing air, acting to maintain constant pressure upon the water; (b) a chamber or cavity filled with air in a boat, airship, etc., to provide or assist buoyancy.

2Biology. In an animal or plant: any chamber or cavity filled with air or gas; especially each of the gas-filled chambers in the shell of a cephalopod.