From WordNet (r) 2.0 (August 2003) [wn]:
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]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Jellyfish \Jel"ly*fish`\, n. (Zool.)
Any one of the acalephs, esp. one of the larger species,
having a jellylike appearance. See Medusa and acaleph.
[1913 Webster]