ommatidium
noun
om·ma·tid·i·um ˌä-mə-ˈti-dē-əm 
plural ommatidia ˌä-mə-ˈti-dē-ə
: one of the elements corresponding to a small simple eye that make up the compound eye of an arthropod
ommatidial
ˌä-mə-ˈti-dē-əl
adjective 
Example Sentences
Recent Examples on the Web In this kind of compound eye, each ommatidium has to be encased in something that blocks light in order to isolate it from the neighboring ommatidia, keeping each unit distinct. 
Scott K. Johnson, Ars Technica, 15 Aug. 2020 At the top of each ommatidium is a lens, with cone cells beneath it that also help focus incoming light. Scott K. Johnson, Ars Technica, 15 Aug. 2020 The tiny organ contains 100 ommatidia, or subunits that make up compound eyes found today in many insects, surrounding a light-sensing structure called a rhabdom. Jason Daley, Smithsonian, 11 Dec. 2017
Scott K. Johnson, Ars Technica, 15 Aug. 2020 At the top of each ommatidium is a lens, with cone cells beneath it that also help focus incoming light. Scott K. Johnson, Ars Technica, 15 Aug. 2020 The tiny organ contains 100 ommatidia, or subunits that make up compound eyes found today in many insects, surrounding a light-sensing structure called a rhabdom. Jason Daley, Smithsonian, 11 Dec. 2017 Word History
Etymology
New Latin, from Greek ommat-, omma eye; akin to Greek ōps eye — more at eye
First Known Use
1884, in the meaning defined above