Entry 1 sense Webster, 1913 Jellyfish /jĕl'-ēf-ĭshˌ/ · Jel·ly·fish · IPA /ˈd͡ʒɛliˌfɪʃ/ 01 n. Any one of the acalephs, esp. one of the larger species, having a jellylike appearance. See Medusa and acaleph. 1. Any one of the acalephs, esp. one of the larger species, having a jellylike appearance. See Medusa and acaleph.(Zool.) See: Medusa, acaleph