means A unit of mass equal to one millionth of a gram, used to measure substances so potent that even a tiny quantity matters.

from A modern scientific compound: the Greek 'mikros' (small) bolted onto 'gram,' which travels back through French 'gramme' to Late Latin 'gramma' and Greek 'gramma' — originally a small written mark or a small weight. So a microgram is, quite literally, a 'tiny little weight,' a name assembled by the metric system's habit of stacking Greek prefixes onto its base units.