cnidarian UK /naɪˈdɛəɹ.i.ən/US /naɪˈdɛɹ.i.ən/adjective 1noun 1forms 3 adjective1 •Of or relating to a cnidarian. noun1 •Any of various invertebrate animals, such as jellyfish, hydras, sea anemones, corals and formerly sponges and ctenophores that belong to the phylum Cnidaria.A general feature of cnidarians, as I noted above, is their stinging cells. — 2016 formscnidarians · more cnidarian · most cnidarian