The smallest basic unit of a compound eye in insects like cockroaches is known as [[1]].
 

The smallest basic unit of a compound eye in insects like cockroaches is known as ommatidium.

Cockroaches are insects belonging to the phylum Arthropoda. Insects and many other arthropods have compound eyes, an organ for vision. Compound eyes are composed of basic units called ommatidia. 

An ommatidium contains a cluster of photoreceptor cells surrounded by support cells and pigment cells, that form several images combined to form a complete vision. This type of vision is known as mosaic vision.