jellyfish

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.

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

n.

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).

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.