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]

Moby Thesaurus II by Grady Ward, 1.0:

100 Moby Thesaurus words for "microorganism":
   Euglena, adenovirus, aerobe, aerobic bacteria, aerobic organism,
   amoeba, anaerobe, anaerobic bacteria, anaerobic organism,
   animalcule, autotrophic organism, bacillus, bacteria, bacterium,
   being, bug, coccus, colon bacillus, creature, crumb, diatom,
   disease-producing microorganism, dot, drop, droplet, dyad,
   echovirus, enterovirus, filterable virus, flagellate, fleck,
   flyspeck, fungus, genetic individual, germ, gnat, grain,
   gram-negative bacteria, gram-positive bacteria,
   heterotrophic organism, individual, iota, jot, living being,
   living thing, microbe, microspore, microzoa, midge, minim, minutia,
   minutiae, mite, mold, monad, morphological individual, mote,
   nematode, nonfilterable virus, ont, organic being, organism,
   organization, paramecium, particle, pathogen,
   physiological individual, picornavirus, pinhead, pinpoint, point,
   protozoa, protozoon, reovirus, rhinovirus, rickettsia, salmonella,
   saprophyte, scrap, snip, snippet, speck, spirillum, spirochete,
   spore, sporozoon, staphylococcus, streptococcus, tetrad, tittle,
   triad, trypanosome, vanishing point, vibrio, virus, volvox,
   vorticellum, zooid, zoon, zoospore