An air-filled sac near the spinal column in many fishes that helps maintain buoyancy WordNet 3.0 & Open English WordNet

The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish to control their buoyancy, and thus to stay at their current water depth without having to expend energy in swimming. Wikipedia

Gas-filled organ that contributes to the ability of a fish to control its buoyancy Wikidata

Internal organ filled with air with which fish can control their buoyancy. OmegaWiki

An internal organ that fish use to control their buoyancy, allowing them to maintain or change depth by changing their density. Wiktionary

A gas-filled sac within the bodies of most fish that variously acts as a float, lung, sound-producing organ or an aid to hearing. Wiktionary

Fish organ. Wiktionary (translation)