The Collaborative International Dictionary of English v.0.48:

Tergum \Ter"gum\, n.; pl. Terga. [L., the back.] (Zool.)
   (a) The back of an animal.
   (b) The dorsal piece of a somite of an articulate animal.
   (c) One of the dorsal plates of the operculum of a cirriped.
       [1913 Webster]