From The Collaborative International Dictionary of English v.0.48:
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.
[1913 Webster]