noun, plural: medusae
(zoology) The freely-swimming, umbrella- or disk-shaped gelatinous form of Cnidarians (as opposed to the polyp form)
Cnidarians exhibit two body forms: the freely swimming medusa and the sessile polyp forms. The medusa appears like an umbrella (others are disk in shape) with trailing tentacles on the lower surface (compare polyp). Also, it reproduces sexually in contrast with polyp reproducing asexually. Scyphozoans are group of Cnidarians that solely exhibit medusa forms. Other Cnidarians such as hydrozoans have both polyp and medusa stages in their life cycle.
Word origin: from its likeness to the tentacled head of Medusa in Greek mythology