In biology, a taxon (plural taxa; back-formation from taxonomy) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and given a particular ranking, especially if and when it is accepted or becomes established. It is not uncommon, however, for taxonomists to remain at odds over what belongs to a taxon and the criteria used for inclusion. If a taxon is given a formal scientific name, its use is then governed by one of the nomenclature codes specifying which scientific name is correct for a particular grouping.
Although preceded by Linnaeus's system in Systema Naturae (10th edition, 1758) and unpublished work by Bernard and Antoine Laurent de Jussieu, the notion of a unit-based "natural system" of biological classification was first made widely available in 1805 through the publication, as the introduction to the third edition of Jean-Baptiste Lamarck's Flore françoise, of Augustin Pyramus de Candolle's Principes élémentaires de botanique, an exposition of a system for the "natural classification" of plants. Since then, systematists have striven to construct an accurate classification encompassing the diversity of life; today, a "good" or "useful" taxon is commonly taken to be one that reflects evolutionary relationships.
The International Association for Plant Taxonomy (IAPT) promotes an understanding of plant biodiversity, facilitates international communication of research between botanists, and oversees matters of uniformity and stability in plant names. The IAPT was founded on July 18, 1950 at the Seventh International Botanical Congress in Stockholm, Sweden. Currently, the IAPT headquarters is located in Bratislava, Slovakia. Its current president, since 2011, is Vicki Funk of the Smithsonian Institution, Washington, DC.; vice-president is Sandra Knapp of the Natural History Museum, London; and secretary-general is Karol Marhold of the Institute of Botany, Slovak Academy of Sciences, Bratislava.
Both the taxonomic journal Taxon and the series Regnum Vegetabile are published by the IAPT. The latter series includes the International Code of Nomenclature for algae, fungi, and plants, the Index Nominum Genericorum, and Index Herbariorum.
The IAPT's primary purpose is the promotion and understanding of biodiversity—the discovery, naming, classification, and systematics of plants—for both living and fossil plants. Additionally, it promotes the study and conservation of plant biodiversity, and works to raise awareness of the general public to this issue. The organization also facilitates international cooperation among botanists working in the fields of plant systematics, taxonomy, and nomenclature. This is accomplished in part through sponsorship of meetings and publication of resources, such as reference publications and journals.