noun, plural: microorganisms
An organism that is microscopic or submicroscopic, which means it is too small to be seen by the unaided human eye.
Microorganisms were first observed by Anton van Leeuwenhoek in 1675 using a microscope of his own design.
Examples of microorganisms include bacteria, fungi, archaea, and protists. Viruses and prions, although microscopic, are not considered microorganisms by others because they are generally regarded as non-living.
Word origin: Gk: comb. form repr. mīkrós small + organism > organ: Gk órganon implement, tool, bodily organ.
Related forms: microorganic, microoganismal (adjective).
See also: organism, microbiology.