monoxide (n.)

word-forming element of Greek origin meaning "one, single, alone; containing one (atom, etc.)," from Greek monos "single, alone," from PIE root *men- (4) "small, isolated."

"compound of oxygen with another element," 1790, from French oxide (1787), coined by French chemists Louis-Bernard Guyton de Morveau and Antoine Lavoisier from ox(ygène) (see oxygen) + (ac)ide "acid" (see acid (n.)).