From WordNet (r) 3.0 (2006) [wn]:
jellyfish
n 1: large siphonophore having a bladderlike float and stinging
tentacles [syn: Portuguese man-of-war, man-of-war,
jellyfish]
2: any of numerous usually marine and free-swimming
coelenterates that constitute the sexually reproductive forms
of hydrozoans and scyphozoans
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]