Source : Webster's Revised Unabridged Dictionary (1913)

Ommatidium \Om`ma*tid"i*um\, n.; pl. {Ommatidia}. [NL., dim. of
   Gr. ?, ?, the eye.] (Zo["o]l.)
   One of the single eyes forming the compound eyes of
   crustaceans, insects, and other invertebrates.

Source : WordNet®

ommatidium
     n : any of the numerous small cone-shaped eyes that make up the
         compound eyes of some arthropods
     [also: {ommatidia} (pl)]