any of several auxiliary air containers within a balloon or airship, that can be inflated or deflated to compensate for changing gas pressure during flight: used to control altitude and maintain proper structure
See ballonet in American Heritage Dictionary 4
(bălˌə-nāˈ)
noun
One of several small auxiliary gasbags placed inside a balloon or a nonrigid airship that can be inflated or deflated during flight to control and maintain shape and buoyancy.