Medusa

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The Gorgon who was killed by Perseus.

from The Century Dictionary.

• noun In Gr. mythology, one of the three Gorgons, the only one of them who was mortal.
• noun Pl. medusæ (-sē). In zoology: [lowercase] A jelly-fish, sea-jelly, or sea-nettle; an acaleph, in a strict sense; a discophoran or discophorous hydrozoan; any member of the family Medusidæ or order or subclass Discophora: a term very loosely used, and now chiefly as an English word. See medusoid, n.
• noun [capitalized] [NL.] An old genus of jelly-fishes, used with great and varying latitude, more or less nearly equivalent to the order Discophora or family Medusidæ, now greatly restricted or entirely discarded. In the latter case Aurelia is used instead. See cut under acaleph. [In this sense there is no plural.]
• noun [lowercase] Some hydrozoan resembling or supposed to be one of the foregoing; a medusoid: as, the naked-eyed medusæ of Forbes, which are the reproductive zoöids or gonophores of gymnoblastic hydroids.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Class. Myth.) The Gorgon; or one of the Gorgons whose hair was changed into serpents, after which all who looked upon her were turned into stone.
• noun (Zoöl.) Any free swimming acaleph; a jellyfish.
• noun (Zoöl.) one of the buds of a hydroid, destined to develop into a gonophore or medusa. See Athecata, and Gonotheca.
• noun (Zoöl.), (Astron.) A cluster of stars in the constellation Perseus. It contains the bright star Algol.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• proper noun Greek mythology : The only mortal of the three gorgon sisters. She is killed by Perseus.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (Greek mythology) a woman transformed into a Gorgon by Athena; she was slain by Perseus
• noun one of two forms that coelenterates take: it is the free-swimming sexual phase in the life cycle of a coelenterate; in this phase it has a gelatinous umbrella-shaped body and tentacles