an air or gas compartment in a balloon or nonrigid airship, used to control buoyancy and shape
Word origin
C20: from French ballonnet a little balloon
ballonet in American English
(ˌbæləˈnɛt)
noun
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