A one-celled microscopic animal included in the kingdom of Protista: A protozoan can be a flagellate, ciliate, sporozoan or amoeba.

A protozoan can be any of a diverse group of eukaryotes (organisms composed of one or more cells, each of which contains a clearly defined nucleus enclosed by a membrane, along with organelles [small, self-contained, cellular parts that perform specific functions]), of the kingdom Protista, that are primarily unicellular, existing singly or aggregating in colonies, are usually nonphotosynthetic, and are often classified further into phyla according to their capacity for and means of motility, as by pseudopods, flagella, or cilia.