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

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "jellyfish":
   Chilopoda, Chordata, Echiuroidea, Ectoprocta, Entoprocta,
   Milquetoast, Monoplacophora, Nemertinea, Phoronidea, baby,
   big baby, chicken, chicken liver, coward, crybaby, doormat,
   dull tool, fence-sitter, fraid-cat, fraidy-cat, funk, funker,
   gutless wonder, invertebrate, lightweight, lily liver, meek soul,
   milksop, mollycoddle, mouse, mugwump, namby-pamby, nebbish,
   nonentity, pansy, pantywaist, pushover, sad sack, scaredy-cat,
   shilly-shally, sissy, softling, softy, sop, waverer, weak sister,
   weakling, white feather, white liver, wobbler