The Collaborative International Dictionary of English v.0.48:

Opisthobranchia \O*pis`tho*bran"chi*a\, Opisthobranchiata
\O*pis`tho*bran`chi*a"ta\, n. pl. [NL., from Gr. 'o`pisqen
   behind + ? gills.] (Zool.)
   A division of gastropod Mollusca, in which the breathing
   organs are usually situated behind the heart. It includes the
   tectibranchs and nudibranchs.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Opisthobranchia
    n 1: gastropods having the gills when present posterior to the
         heart and having no operculum: includes sea slugs; sea
         butterflies; sea hares [syn: Opisthobranchia, subclass
         Opisthobranchia]