jellyfishEnglishOxford New American Dictionaryjellyfish /ˈjelēˌfiSH, ˈdʒɛliˌfɪʃ / ▸ noun (plural same or jellyfishes) 1 a free-swimming marine coelenterate with a gelatinous bell- or saucer-shaped body that is typically transparent and has stinging tentacles around the edge. Classes Scyphozoa and Cubozoa. 2 informal a feeble or weak-willed person.