Marine invertebrate of the phylum Cnidaria (coelenterates) with an umbrella-shaped body composed of a semitransparent gelatinous substance, with a fringe of stinging tentacles. Most adult jellyfishes move freely, but during parts of their life cycle many are polyplike and attached. They feed on small animals that are paralyzed by stinging cells in the jellyfishes' tentacles.
Any of numerous usually marine and free-swimming coelenterates that constitute the sexually reproductive forms of hydrozoans and scyphozoans; SYN. medusa, medusan.