The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

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