Entry 1 sense Webster, 1913 Sea urchin /sēʹ ûr'tsĭn/ · Sea· ur·chin · IPA /ˈsi ˌɝtʃɪ̈n/ 01 Any one of numerous species of echinoderms of the order Echinoidea. 1. Any one of numerous species of echinoderms of the order Echinoidea.(Zool.)