ommatidium

from WordNet (r) 3.0 (2006)

ommatidium
    n 1: any of the numerous small cone-shaped eyes that make up the
         compound eyes of some arthropods

from 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]