Source : Webster's Revised Unabridged Dictionary (1913)

Microorganism \Mi`cro*["o]r"gan*ism\, n. [Micro- + organism.]
   (Biol.)
   Any microscopic form of life; -- particularly applied to
   bacteria and similar organisms, esp. such are supposed to
   cause infectious diseases.

Source : WordNet®

microorganism
     n : any organism of microscopic size