me·du·sa / məˈdoōsə; -zə/ • n. (pl. -sae / -sē; -sī; -zē; -zī/ or -sas ) Zool. a free-swimming sexual form of a coelenterate such as a jellyfish, typically having an umbrella-shaped body with stinging tentacles around the edge. In some species, medusae are a phase in the life cycle that alternates with a polypoid phase. Compare with polyp. ∎  a jellyfish.

medusa The free-swimming stage in the life cycle of the Cnidaria. Medusae are umbrella-shaped, with tentacles round the edge and the mouth in the centre underneath. They swim by pulsations of the body and reproduce sexually. In the Hydrozoa (e.g. Hydra) they alternate in the life cycle with polyps, from which they are produced by budding. In the Scyphozoa, which includes all the common jellyfish, the medusa is the dominant form and the polyp is reduced or absent.

medusa A ‘jellyfish’; in Cnidaria, the free-swimming body type, resembling an umbrella or bell, that floats convex side uppermost. The mouth is located at the centre of the under-side of the bell and tentacles hang from its edge.