protozoan Any memeber of the phylum Protozoa, which comprises unicellular organisms found worldwide in marine or freshwater, free-living and as parasites. These microscopic animals have the ability to move (by cilia or pseudopodia) and have a nucleus, cytoplasm and cell wall; some contain chlorophyll. Reproduction is by fission or encystment. Length: 0.3mm (0.1in). The 30,000 species divide into four classes: Flagellata, Cnidospora, Ciliophora, and Sporozoa.

pro·to·zo·an / ˌprōtəˈzōən/ • n. (pl. -zo·a, -zo·ans ) any single-celled microscopic animal of the phylum Protozoa, including amebas and ciliates.• adj. of or relating to protozoa.