Terminology of protozoa

Following the Greek root of the name, the singular form is protozoon  (protos=first, zoon=animal). Its use has, however, partially been replaced by the word protozoan, which was originally only used as an adjective. In the same manner the plural form protozoans is sometimes being used instead of protozoa.
In general, protozoa are referred to as animal-like proticts because of movement (motility). However, both protozoa and protists are paraphyletic groups (not including all genetic relatives of the group). For example, Entamoeba is more closely related to humans than to Euglena. "Protozoa" is considered an outdated classification in more formal contexts. However, the term is still used in children's education.
While there is no exact definition for the term protozoa, it is often referred to as a unicellular heterotrophic protist such as the amoeba and ciliates. The term algae is used for microorganisms that photosynthesize. However, distinction between protozoa and algae is often vague. For example, the alga Dinobryon has chloroplasts for photosynthesis, but it can also feed on organic matter and is motile.
  Protozoa is sometimes considered a subkingdom It was traditionally considered a phylum under Animalia referring to unicellular animals, with Metazoa referring to multicellular animals.