The Collaborative International Dictionary of English v.0.48:

Microorganism \Mi`cro*or"gan*ism\, n. [Micro- + organism.]
   (Biol.)
   Any microscopic form of life; a form of life too small to be
   seen by the naked eye; -- particularly applied to bacteria,
   protozoa, yeasts, and similar organisms, esp. such are
   supposed to cause infectious diseases.
   [1913 Webster]

WordNet (r) 3.0 (2006):

microorganism
    n 1: any organism of microscopic size [syn: microorganism,
         micro-organism]