protozoan pro·to·zo·an (prō'tə-zō'ən) or pro·to·zo·on (-ŏn')
n. pl. pro·to·zo·a (-zō'ə) or pro·to·zo·ans or pro·to·zo·a or pro·to·zo·ons
Any of a group of single-celled, usually microscopic, eukaryotic organisms, such as amoebas, ciliates, flagellates, and sporozoans.
Plural protozoans or protozoa
Any of a large group of one-celled organisms (called protists) that live in water or as parasites. Many protozoans move about by means of appendages known as cilia or flagella. Protozoans include the amoebas, flagellates, foraminiferans, and ciliates. Their traditional classification as the subkingdom Protozoa is still used for convenience, but it is now known that protozoans represent several evolutionarily distinct groups. See more at protist.