any of a genus (Physalia) of large, colonial, warm-sea siphonophores having a large, bladderlike sac, with a sail-like structure on top, which enables them to float on the water, and long, dangling tentacles that have powerful stinging cells
See Portuguese man-of-war in American Heritage Dictionary 4
noun
A complex colonial siphonophore of the genus Physalia, of warm seas, having a bluish bladderlike float with a broad saillike crest from which hang numerous long stinging tentacles.