Protozoan was historically a kingdom, several levels about a phylum, of single-celled organisms. Most were heterotrophs, deriving energy from organic carbon gained by consuming other organisms. Modern biological taxonomies abandoned the formal nomenclature of this group, as it represents many organisms with divergent evolutionary backgrounds.
Informally, the term is still used descriptively to describe a group of eukaryotes, organisms that possess a membrane-bound nucleus, that lack filamentous structures found in molds. Although most classically-defined protozoans are heterotrophs, some are mixotrophs; these organisms are also capable of autotrophy, the generation of biological energy from chemical metabolism or sunlight. Generally, protozoans are all able to move through aqueous environments by using pseudopods, cilia or flagella to propel themselves.