Noun, pl. facets
1. A smooth circumscribed surface. (wiktionary.org)
1. One member of a compound eye, as found in insects and crustaceans. (wiktionary.org)
2. Any of the individual units (ommatidia) that make up the compound eye of an insect or crustacean. (Google Dictionary)
Word origin: From French facette (12c., Old French facete), diminutive of face. The diamond-cutting sense is the original one.