'Microorganism' definitions:

Definition of 'microorganism'

(from WordNet)
noun
Any organism of microscopic size [syn: microorganism, micro-organism]

Definition of 'Microorganism'

From: GCIDE
  • 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]

Synonyms of 'microorganism'

From: Moby Thesaurus