ommatidium
Concise Oxford English Dictionary © 2008 Oxford University Press:
ommatidium /ˌɒməˈtɪdɪəm/
▶noun (pl. ommatidia /-dɪə/) Entomology each of the optical units that make up the compound eye of an insect.
– origin C19: mod. L., from Gk ommatidion, dimin. of omma, ommat- ‘eye’.
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