Protozoa: a now obsolete term that referred to colorless protists and included flagellates, ciliates, amoebae and sporozoa. The protozoa were all thought to be single celled eukaryotes. The concept was popular for much of the 20th century, but it has become clear that some are relatives of animals and fungi, others have closer relationships with algae than with other protozoa, and that protozoa are not closely related. The term has largely fallen into disuse.