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In biological taxonomy, a kingdom or regnum is a taxon in either (historically) the highest rank, or (in the new three-domain system) the rank below domain. Each kingdom is divided into smaller groups called phyla (or in some contexts these are called divisions). Currently, textbooks from the United States use a system of six kingdoms (Animalia, Plantae, Fungi, Protoctista, Archaea, and Monera), while British and Australian textbooks describe five kingdoms (Animalia, Plantae, Fungi, Protista, Bacteria).
Carolus Linnaeus distinguished two kingdoms of living things. Animalia for animals and Vegetabilia for plants (Linnaeus also treated minerals, placing them in a third kingdom, Mineralia). Linnaeus divided each kingdom into classes, later grouped into phyla for animals and divisions for plants. When single-celled organisms were first discovered, they were split between the two kingdoms, mobile forms in the animal phylum protozoa, and colored algae and bacteria in the plant division Thallophyta or Protophyta. However, a number of forms were hard to place, or were placed in different kingdoms by different authors for example, the mobile alga Euglena and the amoeba-like slime moulds. As a result, Ernst Haeckel suggested creating a third kingdom Protista for them.