Definition from Wiktionary, the free dictionary
Alternative forms 
microorganism (plural microorganisms)
- (microbiology) An organism that is too small to be seen by the unaided eye, especially a single-celled organism, such as a bacterium.
- The cud is then reswallowed and further digested by specialized microorganisms that live in the rumen.
Related terms 
an organism too small to be seen by unaided eye