Taxonomy is the practice and science of classification. The word finds its roots in the Greek τάξις, taxis (meaning 'order' or 'arrangement') and νόμος, nomos (meaning 'law' or 'science'). Taxonomy uses taxonomic units, known as taxa (singular taxon).
In addition, the word is also used as a count noun: a taxonomy, or taxonomic scheme, is a particular classification ("the taxonomy of ..."), arranged in a hierarchical structure. Typically this is organized by supertype-subtype relationships, also called generalization-specialization relationships, or less formally, parent-child relationships. In such an inheritance relationship, the subtype by definition has the same properties, behaviors, and constraints as the supertype plus one or more additional properties, behaviors, or constraints. For example: car is a subtype of vehicle, so any car is also a vehicle, but not every vehicle is a car. Therefore a type needs to satisfy more constraints to be a car than to be a vehicle. Another example: any shirt is also a piece of clothing, but not every piece of clothing is a shirt. Hence, a type must satisfy more parameters to be a shirt than to be a piece of clothing.
Originally taxonomy referred only to the classifying of organisms (now sometimes known as alpha taxonomy) or a particular classification of organisms. It is also used to refer a classification of things or concepts, as well as to the principles underlying such a classification.
Almost anything—animate objects, inanimate objects, places, concepts, events, properties, and relationships—may then be classified according to some taxonomic scheme. Wikipedia categories illustrate a taxonomy schema, and a full taxonomy of Wikipedia categories can be extracted by automatic means. Recently, it has been shown that a manually constructed taxonomy, such as that of computational lexicons like WordNet, can be used to improve and restructure the Wikipedia category taxonomy.
The term taxonomy is sometimes applied to relationship schemes other than parent-child hierarchies, such as network structures with other types of relationships. In that case, they might include single children with multi-parents, for example, "Car" might appear with both parents "Vehicle" and "Steel Mechanisms"; technically, this merely means that 'car' is a part of several different taxonomies. A taxonomy might also be a simple organization of kinds of things into groups, or even an alphabetical list. However, the term vocabulary is more appropriate for such a list. In current usage within Knowledge Management, taxonomies are considered narrower than ontologies since ontologies apply a larger variety of relation types.
Mathematically, a hierarchical taxonomy is a tree structure of classifications for a given set of objects. It is also named Containment hierarchy. At the top of this structure is a single classification, the root node, that applies to all objects. Nodes below this root are more specific classifications that apply to subsets of the total set of classified objects. The progress of reasoning proceeds from the general to the more specific. In scientific taxonomies, a conflative term is always a polyseme.
In contrast, in a context of legal terminology, an open-ended contextual taxonomy—a taxonomy holding only with respect to a specific context. In scenarios taken from the legal domain, a formal account of the open-texture of legal terms is modeled, which suggests varying notions of the "core" and "penumbra" of the meanings of a concept. The progress of reasoning proceeds from the specific to the more general.
Some have argued that the adult human mind naturally organizes its knowledge of the world into such systems. This view is often based on the epistemology of Immanuel Kant. Anthropologists have observed that taxonomies are generally embedded in local cultural and social systems, and serve various social functions. Perhaps the most well-known and influential study of folk taxonomies is Émile Durkheim's The Elementary Forms of Religious Life. A more recent treatment of folk taxonomies (including the results of several decades of empirical research) and the discussion of their relation to the scientific taxonomy can be found in Scott Atran's Cognitive Foundations of Natural History
Biological classification (sometimes known as "Linnaean taxonomy") is still generally the best known form of taxonomy. It differs from the above in that it is an empirical science, with classifying only the final step of a process, and a classification only the means to communicate the end results. It also includes the prediction, discovery, description and (re)defining of taxa. It uses taxonomic ranks, including, among others, (in order) Kingdom, Phylum, Class, Order, Family, Genus, Species (various mnemonic devices have been used to help people remember the list of "Linnaean" taxonomic ranks. See Zoology mnemonic). In zoology, the nomenclature for the more important ranks (superfamily to subspecies), including the allowed number of ranks, is strictly regulated by the ICZN Code, whereas there is more latitude for names at higher ranks. Taxonomy itself is never regulated, but is always the result of research in the scientific community. How researchers arrive at their taxa varies; depending on the available data, and resources, methods vary from simple quantitative or qualitative comparisons of striking features to elaborate computer analyses of large amounts of DNA sequence data.
Today, the alternative to the traditional rank-based biological classification is phylogenetic systematics, which is postulating phylogenetic trees (trees of descent), rather than focusing on what taxa to delimit. The best-known form of this is cladistics.
The results of cladistic analyses are often represented as cladograms. It is held by cladists that taxa (if recognized) must always correspond to clades, united by apomorphies (derived traits) which are discovered by a cladistic analysis. Some cladists[who?] hold that clades are poorly expressed in rank-based hierarchies and support the PhyloCode, a proposed ruleswork for the formal naming of clades, based on the model of the ICZN, ICBN etc. in rank-based nomenclature.
In numerical taxonomy, numerical phenetics or taximetrics, the taxonomy is exclusively based on cluster analysis and neighbor joining to best-fit numerical equations that characterize measurable traits of a number of organisms. It results in a measure of evolutionary "distance" between species. This method has been largely superseded by cladistic analyses today; it is liable to being misled by plesiomorphic traits.
Other taxonomies, such as those analyzed by Durkheim and Lévi-Strauss, are sometimes called folk taxonomies to distinguish them from scientific taxonomies that focus on evolutionary relationships rather than similarity in habitus and habits. Though phenetics arguably places much emphasis on overall similarity, it is a quantitative analysis that attempts to reproduce evolutionary relationships of lineages and not similarities of form taxa.
The neologism folksonomy should not be confused with "folk taxonomy", though it is obviously a portmanteau created from the two words. "Fauxonomy" (from French faux, "false") is a pejorative neologism used to criticize folk taxonomies for their lack of agreement with scientific findings. Baraminology is a taxonomy used in creation science which in classifying form taxa resembles folk taxonomies.
The phrase "enterprise taxonomy" is used in business to describe a very limited form of taxonomy used only within one organization. An example would be a certain method of classifying trees as "Type A", "Type B" and "Type C" used only by a certain lumber company for categorising log shipments.
Military theorist Carl von Clausewitz stressed the significance of grasping the fundamentals of any situation in the "blink of an eye" (coup d'œil). In a military context the astute tactician can immediately grasp a range of implications and can begin to anticipate plausible and appropriate courses of action. Clausewitz' conceptual "blink" represents a tentative ontology which organizes a set of concepts within a domain.
The term "military taxonomy" encompasses the domains of weapons, equipment, organizations, strategies, and tactics. The use of taxonomies in the military extends beyond its value as an indexing tool or record-keeping template -- for example, the taxonomy-model analysis suggests a useful depiction of the spectrum of the use of military force in a political context.
A taxonomy of terms to describe various types of military operations is fundamentally affected by the way all elements are defined and addressed—not unlike framing. For example, in terms of a specific military operation, a taxonomic approach based on differentiation and categorization of the entities participating would produce results which were quite different from an approach based on functional objective of an operation (such as peacekeeping, disaster relief, or counter-terrorism).
Taxonomies are also often used to classify economic activity, including products, companies and industries.
Widely used industry taxonomies include the International Standard Industrial Classification (ISIC); national and regional taxonomies such as the United States Standard Industrial Classification (SIC), the North American Industry Classification System (NAICS), Statistical classification of economic activities in the European Community (NACE), the United Kingdom Standard Industrial Classification of Economic Activities, the Russian Economic Activities Classification System (OKVED); and proprietary taxonomies such as the Industry Classification Benchmark and Global Industry Classification Standard. The international and national taxonomies are used by official statistical agencies. The proprietary taxonomies are often used in the financial services industry to group similar investment vehicles and to construct sectorial stock market indices.
Pavitt's Taxonomy classifies firms by their principal sources of innovation.
The creation of taxonomies is very important in safety science. For example there exist numerous taxonomies to classify and analyze human error and accident causes. Examples of these include the Human Factors Analysis and Classification System based on Reason's Swiss Cheese Model, the CREAM (Cognitive Reliability Error Analysis Method), the taxonomy used by CIRAS (Confidential Incident Railway Analysis System) in the UK rail industry, and others.