air chamber

from The American Heritage® Dictionary of the English Language, 5th Edition.

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

from the GNU version of the Collaborative International Dictionary of English.

• A chamber or cavity filled with air, in an animal or plant.
• A cavity containing air to act as a spring for equalizing the flow of a liquid in a pump or other hydraulic machine.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a chamber, in a hydraulic system, in which air (or some other gas) is compressed to regulate the flow of a fluid