The Collaborative International Dictionary of English v.0.48:

Ommatidium \Om`ma*tid"i*um\, n.; pl. Ommatidia. [NL., dim. of
   Gr. ?, ?, the eye.] (Zool.)
   One of the single eyes forming the compound eyes of
   crustaceans, insects, and other invertebrates; one of the
   eyes of an ommateum.
   [1913 Webster]