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]