Meaning

noun (chemistry) an atom having a valence of one

noun a singular metaphysical entity from which material properties are said to derive

noun (biology) a single-celled microorganism (especially a flagellate protozoan)

originFrom Latin monas (“unit”) (from Ancient Greek μονάς (monás), from μόνος (mónos), from Proto-Indo-European *men-). By surface analysis, mono- + -ad.

Etymology from Wiktionary, CC BY-SA 3.0