Protozoa (also protozoan, plural protozoans) is an informal term for single-celled eukaryotic organisms, either free-living or parasitic, which feed on organic matter such as other microorganisms or organic tissues and debris.

Historically, the protozoa were regarded as “one-celled animals,” because they often possess animal-like behaviors, such as motility and predation, and lack a cell wall, as found in plants and many algae.

Although the traditional practice of grouping of protozoa with animals is no longer considered valid, the term continues to be used in a loose way to identify single-celled organisms that can move independently and feed by heterotrophy.