Kingdom

Definition
noun, plural: kingdoms
In biology, kingdom is a taxonomic rank that is composed of smaller groups called phyla (or divisions, in plants).
Supplement
Historically, kingdom is the highest taxonomic rank, or the most general taxon used in classifying organisms. However, in the new three-domain system introduced by Carl Woese in 1990, the domain is the most general taxon, and kingdom is only next.
The five-kingdom taxonomic classification of the world’s biota into Kingdom Animalia, Plantae, Fungi, Protista, and Monera as proposed by Robert Whittaker in 1969 has become a popular standard of classifying organisms. It became the basis for newer multi-kingdom systems such as the six-kingdom system of Carl Woese and colleagues in 1977.
The five biological Kingdoms (by Robert Whittaker):
Kingdom Monera: the most primitive of the five kingdoms that includes all the bacteria, also called monerans, which are single-celled prokaryotic organisms. (In six-kingdom system, Kingdom Monera is split into two kingdoms: (1) the Eubacteria, which are all bacteria apart from the archaebacteria, and (2) the Archaebacteria, which are single-celled organisms that live under extreme environmental conditions and have distinctive biochemical features)
Kingdom Protista: composed of single-celled and multicellular eukaryotes without the highly specialized tissues. Protists include protozoa and some types of algae
Kingdom Fungi: includes multicellular, non-photosynthetic, saprotrophic organisms such as slime moulds, mushrooms, smuts, rusts, mildews, moulds, stinkhorns, puffballs, truffles and yeasts that absorb food in solution directly through their cell walls and reproduce through spores
Kingdom Plantae: members of this kingdom are multicellular, (mostly) autotrophic eukaryotes that (usually) conduct photosynthesis
Kingdom Animalia: members of this kingdom are multicellular, heterotrophic eukaryotes that digest food outside their cells and then absorb the digested nutrients
Word origin: from Old English cyningdōm : cyning, king; see king + -dōm.
See also: taxonomy.