A botanical name is a formal scientific name conforming to the International Code of Botanical Nomenclature (ICBN) and, if the plant is a cultigen, the additional cultivar and/or Group epithets must conform to the International Code of Nomenclature for Cultivated Plants (ICNCP). The purpose of a formal name is to have a single name that is accepted and used worldwide for a particular plant or plant group. For example, the botanical name Bellis perennis is used worldwide for a plant species, which is native to and has a history of many centuries use in most of the countries of Europe and the Middle East, where it has accumulated various names in the many languages of that area. Later it has been introduced worldwide, bringing it into contact with languages on all continents. The cultivar Bellis perennis 'Aucubifolia' is a golden-variegated horticultural selection of this species. English names for this plant species include: daisy, common daisy, lawndaisy, etc.
The usefulness of botanical names is limited by the fact that taxonomic groups are not fixed in size; a taxon may have a varying circumscription. The group that a particular botanical name refers to can be quite small according to some people and quite big according to others. This will depend on taxonomic viewpoint or taxonomic system. The traditional view of the family Malvaceae includes over a thousand species, but in some modern approaches it contains over four thousand species. The botanical name itself is fixed by a type, the size and placement of the taxon it applies to is set by a taxonomist. Some botanical names refer to groups that are very stable (for example Equisetaceae, Magnoliaceae) while for other names a careful check is needed to see which circumscription is being used (for example Fabaceae, Scrophulariaceae, Urticaceae, etc).
Depending on rank, botanical names may be in one part (genus and above), two parts (species and above, but below the rank of genus) or three parts (below the rank of species):
in one part
Plantae (the plants)
Marchantiophyta (the liverworts)
Magnoliopsida (class including the family Magnoliaceae)
Liliidae (subclass including the family Liliaceae)
Pinophyta (the conifers)
Fagaceae (the beech family)
Betula (the birch genus)
in two parts
Acacia subg. Phyllodineae (the wattles)
Berberis thunbergii (Japanese barberry)
in three parts
Calystegia sepium subsp. americana (American hedge bindweed)
A name in three parts, i.e. the name of an infraspecific taxon (below the rank of species) needs a "connecting term" to indicate rank. In the Calystegia example above, this is "subsp." (for subspecies). In botany there are many ranks below that of species (in zoology there is only one such rank, subspecies, so that this "connecting term" is unnecessary there). A name of a "subdivision of a genus" also needs a connecting term (in the Acacia example above, this is "subg.", subgenus). The connecting term is not part of the name itself.
A taxon may be indicated by a listing in more than three parts: "Saxifraga aizoon var. aizoon subvar. brevifolia f. multicaulis subf. surculosa Engl. & Irmsch." but this is a classification, not a formal botanical name. The botanical name is Saxifraga aizoon subf. surculosa Engl. & Irmsch. (ICBN).
Generic, specific, and infraspecific botanical names are usually printed in italics. The example set by the ICBN is to italicize all botanical names, including those above genus, though the ICBN preface states: "The Code sets no binding standard in this respect, as typography is a matter of editorial style and tradition not of nomenclature". Most peer-reviewed scientific botanical publications do not italicize names above the rank of genus, and non-botanical scientific publications do not, which is in keeping with two of the three other kinds of scientific name; zoological and bacterial (viral names above genus are italicized, a new policy adopted in the early 1990s).
For botanical nomenclature, the ICBN prescribes a two-part name or binary name for any taxon below the rank of genus down to, and including the rank of species. Taxa below the rank of species (infraspecific taxa) get a three part name (ternary name).
The ranks explicitly mentioned in the ICBN as having a binary name are:
A binary name consists of the name of a genus and an epithet.
⇒ In the case of a species this is a specific epithet:
Bellis perennis is the name of a species, with perennis the specific epithet. There is no connecting term involved, to indicate the rank
⇒ In the case of a subdivion of a genus (subgenus, section, subsection, series, subseries, etc) the name consists of the name of a genus and a subdivisional epithet. A connecting term should be placed before the subdivisional epithet to indicate the rank.
More than two parts
⇒ In the case of a cultivar there is an additional cultivar epithet (this is a non-Latin part of the botanical name and is not written in italics). The cultivar epithet may follow either the botanical name of the species, or the name of the genus only, or the common name of the genus or species (provided the common name is unambiguous). The generic name, followed by the cultivar name, is often used when the parentage of a particular hybrid cultivar is uncertain or when it cannot be linked with certainty to a particular species.
Bellis perennis is the name of a species, with perennis the specific epithet and 'Aucubifolia' the cultivar epithet.
⇒ Sometimes a subdivision of a genus may be indicated with a listing in three or more parts. However, this is not its formal name. Its botanical name is in two parts.
⇒ Classification of Kingdom Plantae
⇒ Scientific classification
⇒ Binomial nomenclature
⇒ The International Code of Nomenclature for Cultivated Plants
⇒ The International Code of Botanical Nomenclature