In biology, kingdom (Latin: regnum, plural regna) is the second highest taxonomic rank, just below domain. Kingdoms are divided into smaller groups called phyla. Traditionally, some textbooks from the United States used a system of six kingdoms (Animalia, Plantae, Fungi, Protista, Archaea/Archaeabacteria, and Bacteria/Eubacteria) while textbooks in Great Britain, India, Australia, Latin America and other countries used five kingdoms (Animalia, Plantae, Fungi, Protista and Monera). Some recent classifications based on modern cladistics have explicitly abandoned the term "kingdom", noting that the traditional kingdoms are not monophyletic, i.e., do not consist of all the descendants of a common ancestor. 

When Carolus Linnaeus introduced the rank-based system of nomenclature into biology in 1735, the highest rank was given the name "kingdom" and was followed by four other main or principal ranks: class, order, genus and species. Later two further main ranks were introduced, making the sequence kingdom, phylum or division, class, order, family, genus and species. In the 1960s a rank was introduced above kingdom, namely domain (or empire), so that kingdom is no longer the highest rank. 

Prefixes can be added so "subkingdom" ("subregnum") and "infrakingdom" (also known as "infraregnum") are the two ranks immediately below kingdom. Superkingdom may be considered as an equivalent of domain or empire or as an independent rank between kingdom and domain or subdomain. In some classification systems the additional rank "branch" (Latin: "ramus") can be inserted between subkingdom and infrakingdom (e.g. Protostomia and Deuterostomia in the classification of Cavalier-Smith).
Was kingdom the highest rank at first?
When Carolus Linnaeus introduced the rank-based system of nomenclature into biology in 1735, the highest rank was given the name "kingdom" 