A denier was a small coin of French origin made from the eighth century until the French Revolution. Originally it was silk that was measured by its weight as compared to the denier coin. Denier came to be defined as the weight of a certain length of filament fiber or yarn (its linear density) which is 1 gram per 9000 meter length, although there have been variations. Now not only silk is measured by denier, but also manufactured fibers.
A microfiber denier is less than 1, silk filament fiber is about 1 denier. The denier of yarns (as compared to single fibers) ranges from 5 for very sheer hosiery to hundreds for heavy yarns used for making carpet.