Capped piece of pipe, close to a plumbing fixture, which is partially filled with air Also called air cushions they are designed to absorb shock from abrupt water stoppage, which creates a noise known as water hammer
hole that contains air to serve as a spring to equalize the flow of a liquid in a pump or other hydraulic devices; closure of area filled with air for a special objective; chamber or empty space filled with air (in animals or plants)