• (n.) A hollow cylinder to contain air. It is closed above and open below, and has its open end plunged into water.
Air·om'e·ter noun [ Air + - meter .] A hollow cylinder to contain air. It is closed above and open below, and has its open end plunged into water. 
A device for measuring the air content of concrete and mortar.
a small, sensitive anemometer of the windmill type.