- any organism too small to be viewed by the unaided eye, as bacteria, protozoa, and some fungi and algae.
- any organism, such as a bacterium, protozoan, or virus, of microscopic size
- An organism of microscopic or submicroscopic size, especially a bacterium or protozoan.
- An organism that can be seen only with the aid of a microscope and that typically consists of only a single cell. Microorganisms include bacteria, protozoans, and certain algae and fungi. See Note at germ.