Definition: Any minute organism. As originated, the word was intended as a collective term for the large variety of microorganisms then known in the 19th century; modern usage has retained the original collective meaning but expanded it to include both microscopic and ultramicroscopic organisms (spirochetes, bacteria, rickettsiae, and viruses). These organisms are considered to form a biologically distinctive group, in that the genetic material is not surrounded by a nuclear membrane, and mitosis does not occur during replication.
[Fr., fr. G. mikros, small, + bios, life]