In biology, binomial nomenclature is a standard convention used for naming species. As the word 'binomial' suggests, the scientific name of each species is the combination of two names: the genus name and the species epithet. The name of the genus (generic name) is always capitalized, while the specific epithet (trivial name) is not; both are usually typeset in italics, e.g. Homo sapiens. The genus name is usually abbreviated to its initial letter when several species from the same genus are being listed, or discussed in a single paragraph; in a few cases this abbreviation has spread to more general use - for example the bacteriumEscherichia coli is usually referred to as E. coli.
The procedures associated with establishing binomial nomenclature tend to favor stability. In particular, when species are transferred between genera (as not uncommonly happens as a result of new knowledge), wherever possible species names are kept the same. Similarly if what were previously thought to be distinct species are found to belong to a single species, former species names are when applicable retained as a lower taxon name.
However, such stability as exists is far from being absolute. A single organism may have several scientific names in circulation, depending on opinion (see synonymy), conservation according to nomenclature codes, and new findings based on molecular phylogeny.
Another source of instability is the rule that nomenclature should
respect priority of discovery.
In botany, a species can be further divided into any of subspecies, variety, subvariety or form, whereas in zoology, a species is only subdivided into subspecies. Trinomial names of plants therefore usually include a qualifier (such as "subvar." in the example above), whereas trinomial names of animals never do. For example, Phalacrocorax carbo novaehollandiae is the Black Shag, the subspecies of the Great Cormorant found in Australia and New Zealand, and there is no need to indicate explicitly that novaehollandiae is a subspecies name.
Carolus Linnaeus invented this classification, but it is a common misconception that he also invented binomial nomenclature; in fact it dates back to the Bauhins. Linnaeus, however, was the first to systematize and popularize it, and it is only one aspect of his systematical achievements or misachievements (such as oversimplifying fungal systematics).
Binomial nomenclature is only one of many conventions used to name organisms. Nomenclature codes rule the naming of plants (incl. Fungi, cyanobacteria) / cultivated plants / animals / bacteria / viruses. These codes differ. For example, the ICBN plant nomenclature does not allow tautonymy, whereas the ICZN animal code allows it. A BioCode has been suggested to replace several codes, but there also is debate of a PhyloCode to name clades of phylogenetic trees.