Ommatidium n. : One of the single eyes forming the compound eyes of crustaceans, insects, and other invertebrates.

Example:


Taxonomy
ommatidium, ommatidia, pigment cells, free encyclopedia, elements, the eye, cells, the brain, pigment cell, transparent cornea, cross section, species, compound eye, support cells, picture element, search, Unsourced, horsefly.

Defination from the web:
ommatidium collectively form a light guide, a transparent tube, called the rhabdom. ... This has required the rewiring of the eye such that each ommatidium now has seven axons leading from it. ...