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In 21st-century systems of biological classification, the Protozoa [1] are defined as a diverse group of unicellular eukaryotic organisms.Historically, protozoa were defined as single-celled animals or organisms with animal-like behaviors, such as motility and predation. The group was regarded as the zoological counterpart to the "protophyta", which were considered to be plant-like, as they are capable of photosynthesis.

The terms protozoa and protozoans are now mostly used informally to designate single-celled, non-photosynthetic protists, such as the ciliates, amoebae and flagellates.

The term Protozoa was introduced in 1818 for a taxonomic class, but in later classification schemes the group was elevated to higher ranks, including phylum, subkingdom and kingdom. In several classification systems proposed by Thomas Cavalier-Smith and his collaborators since 1981, Protozoa is ranked as a kingdom. The seven-kingdom scheme proposed by Ruggiero et al. in 2015, places eight phyla under Protozoa: Euglenozoa, Amoebozoa, Metamonada, Choanozoa, Loukozoa, Percolozoa, Microsporidia and Sulcozoa. This kingdom does not form a clade, but an evolutionary grade or paraphyletic group, from which the fungi and animals are specifically excluded.

The use of Protozoa as a formal taxon has been discouraged by some recent researchers, mainly because the term, which is formed from the Greek protos "first" + zoia, plural of zoion, "animal", implies kinship with animals (metazoa) and promotes a separation of "animal-like" from "plant-like" organisms, neither of which they encourage. Modern ultrastructural, biochemical, and genetic techniques have shown that protozoa, as traditionally defined, belong to widely divergent lineages, and can no longer be regarded as "primitive animals". For this reason, the terms "protists", "Protista" or "Protoctista" are sometimes preferred for the high-level classification of eukaryotic microbes. In 2005, members of the Society of Protozoologists voted to change the name of that organization to the International Society of Protistologists.