The Collaborative International Dictionary of English v.0.48:

Cnidaria \Cni*da"ri*a\ (n[i^]*d[=a]"r[i^]*[.a]), n. pl. [NL. See
   Cnida.] (Zool.)
   A comprehensive group equivalent to the true C[oe]lenterata,
   i. e., exclusive of the sponges. They are so named from
   presence of stinging cells (cnidae) in the tissues. See
   Coelenterata.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Cnidaria
    n 1: hydras; polyps; jellyfishes; sea anemones; corals [syn:
         Cnidaria, phylum Cnidaria, Coelenterata, phylum
         Coelenterata]