[Cnidaria] As a noun, the sexual stage in the leptolid life-cycle. Fundamentally it comprises a free-living medusa but in many species it is retained on the polypoid stage (i.e., on the hydroid colony) in a variously reduced vestigial state. Reduction is partial, with radial canals and tentacles still discernible, in some species, but in many others is carried further and the medusa stage is reduced to a gonophore (which see) in which the medusa ancestry is evident only by analogy.

As an adjective, medusa-like.