The Collaborative International Dictionary of English v.0.48:

Medusa \Me*du"sa\, n. [L., fr. Gr. ?.]
   1. (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.
      [1913 Webster]

   2. [pl. Medusae.] (Zool.) Any free swimming acaleph; a
      jellyfish.
      [1913 Webster]

   Note: The larger medusae belong to the Discophora, and are
         sometimes called covered-eyed medusae; others, known
         as naked-eyed medusae, belong to the Hydroidea, and
         are usually developed by budding from hydroids. See
         Discophora, Hydroidea, and Hydromedusa.
         [1913 Webster]

   Medusa bud (Zool.), one of the buds of a hydroid, destined
      to develop into a gonophore or medusa. See Athecata, and
      Gonotheca.

   Medusa's head.
      (a) (Zool.) An astrophyton.
      (b) (Astron.) A cluster of stars in the constellation
          Perseus. It contains the bright star Algol.
          [1913 Webster]

WordNet (r) 3.0 (2006):

Medusa
    n 1: (Greek mythology) a woman transformed into a Gorgon by
         Athena; she was slain by Perseus
    2: 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 [syn: medusa, medusoid, medusan]