found: Britannica, viewed December 10, 2021(protozoan, organism, usually single-celled and heterotrophic (using organic carbon as a source of energy), belonging to any of the major lineages of protists and, like most protists, typically microscopic. All protozoans are eukaryotes and therefore possess a "true," or membrane-bound, nucleus. Many are symbionts of other organisms, and some species are parasites. Today the term protozoan is used informally in reference to nonfilamentous heterotrophic protists.)
