Ballonet is a technical term that refers to a gas-filled, inflatable bag that is used in certain types of aircraft. However, there are several synonyms that can be used to describe this concept, including gasbag, bladder, cushion, and envelope. These words all refer to a flexible, air-filled container that can be used to provide buoyancy or support in different contexts. Other synonyms for ballonet might include balloon, airship, dirigible, blimp, or aerostat, depending on the specific type of aircraft or application in question. Whatever term you prefer to use, it's clear that ballonets and other buoyancy technologies have played a critical role in aviation and transportation throughout history.