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noun, plural (especially collectively) jel·ly·fish, (especially referring to two or more kinds or species) jel·ly·fish·es.
  1. any of various marine coelenterates of a soft, gelatinous structure, especially one with an umbrellalike body and long, trailing tentacles; medusa.
  2. Informal. a person without strong resolve or stamina; an indecisive or weak person.

Origin of jellyfish

1700–10; 1910–15 for def 2; jelly + fish Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

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British Dictionary definitions for jellyfish


noun plural -fish or -fishes
  1. any marine medusoid coelenterate of the class Scyphozoa, having a gelatinous umbrella-shaped body with trailing tentacles
  2. any other medusoid coelenterate
  3. informal a weak indecisive person
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for jellyfish

popular name of the medusa and similar sea-creatures, 1796, from jelly (n.) + fish (n.). Earlier it had been used of a type of actual fish (1707).

Online Etymology Dictionary, © 2010 Douglas Harper

jellyfish in Medicine


n. pl. jellyfish
  1. Any of numerous marine coelenterates of the class Scyphozoa, some poisonous species of which, notably the Portuguese man-of-war, produce a toxin that can be injected into the skin by nematocysts on the tentacles, causing linear wheals.
  2. Any of various similar or related coelenterates.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.