air chamber

1. An enclosure filled with air for a special purpose.
2. A chamber, especially in a hydraulic system, in which air elastically compresses and expands to regulate the flow of a fluid.

In a water piping system near a valve or faucet, a vertical pipe stub which is sealed at the top and contains air; the entrapped air provides a cushion when the valve is closed suddenly, thereby eliminating the noise of water hammer.