jellyfish

noun

jel·​ly·​fish ˈje-lē-ˌfish 

1

a

: the typically free-swimming, bell-shaped, usually sexually-reproducing solitary or colonial form of a cnidarian in which the whorls of tentacles lined with nematocysts arise and hang down from the margin of the nearly transparent, gelatinous bell : medusa

especially : a large medusa characteristic of the siphonophores and scyphozoans (such as the sea nettle or box jellyfish)

b

: siphonophore

c

: ctenophore

2

: a person lacking backbone or firmness

Illustration of jellyfish

• jellyfish 1a

Synonyms

• doormat
• namby-pamby
• pushover
• reed
• weakling
• wimp

Examples of jellyfish in a Sentence

a jellyfish who was afraid to tell her boss that her latest brainstorm was just plain bad

Recent Examples on the Web

Research published in 2023 found that a year after test seabed mining disturbed the ocean floor in Japanese waters, the density of fish, crustaceans and jellyfish in nearby areas was cut in half. —Ian Urbina, Los Angeles Times, 28 Mar. 2025

For delicate creatures such as jellyfish and siphonophores – gelatinous animals that can grow over 100 feet long – sediment accumulation can interfere with buoyancy and survival. —Alexus Cazares-Nuesser, The Conversation, 25 Mar. 2025

But, somehow, life finds a way: The seafloor was crawling with critters of all shapes and sizes, from centuries-old sponges and cup corals to octopuses, snails, worms, sea spiders, icefish and even a rare giant phantom jellyfish. —Sarah Kuta, Smithsonian Magazine, 21 Mar. 2025

The water is a stunning aquamarine, crystal-clear, and free from algae or jellyfish. —Lydia Mansel, Travel + Leisure, 19 Mar. 2025

Word History

First Known Use

1818, in the meaning defined at sense 1a

Phrases Containing jellyfish

• box jellyfish

Rhymes for jellyfish

• cuttlefish
• sablefish
• bluefish
• catfish
• codfish
• crawfish
• dogfish
• finfish
• flatfish
• goldfish
• groundfish
• kingfish

Kids Definition

jellyfish

noun

jel·​ly·​fish ˈjel-ē-ˌfish 

1

: any of numerous free-swimming coelenterate animals that reproduce sexually and have a jellylike, saucer-shaped, and usually nearly transparent body and tentacles with stinging cells

2

: any of various sea animals that resemble a jellyfish

Medical Definition

jellyfish

noun

jel·​ly·​fish ˈjel-ē-ˌfish 

: a free-swimming marine coelenterate that is the sexually reproducing form of a hydrozoan or scyphozoan and has a nearly transparent saucer-shaped body and extensible marginal tentacles studded with stinging cells