Source : Webster's Revised Unabridged Dictionary (1913)

Jellyfish \Jel"ly*fish`\, n. (Zo["o]l.)
   Any one of the acalephs, esp. one of the larger species,
   having a jellylike appearance. See {Medusa}.

Source : WordNet®

jellyfish
     n 1: large siphonophore having a bladderlike float and stinging
          tentacles [syn: {Portuguese man-of-war}, {man-of-war}]
     2: any of numerous usually marine and free-swimming
        coelenterates that constitute the sexually reproductive
        forms of hydrozoans and scyphozoans [syn: {medusa}, {medusan}]
     [also: {jellyfishes} (pl)]