Micro- \Mi"cro-\, Micr- \Mi"cr-\ . [Gr. mikro`s small.]

A combining form signifying:
   (a) Small, little, trivial, slight; as, microcosm, microscope.
   (b) (Metric System, Elec., Mech., etc.) A millionth part of; as, microfarad, microohm, micrometer.

Source: Webster's Revised Unabridged Dictionary (1913)