Taxonomy is the practice and science of classification. The word finds its roots in the Greek τάξις, taxis (meaning ‘order‘, ‘arrangement‘) and νόμος, nomos (‘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 organised by subtype-supertype relationships, also called parent-child relationships. In such a subtype-supertype relationship the subtype kind of thing has by definition the same constraints as the supertype kind of thing plus one or more additional 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 thing needs to satisfy more constraints to be a car than to be a vehicle.


Originally the term taxonomy referred only to the classifying of organisms (now sometimes known as alpha taxonomy) or a particular classification of organisms. However, it has become fashionable in certain circles to apply the term in a wider, more general sense, where it may refer to 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 be then classified according to some taxonomic scheme

In an even wider sense, the term taxonomy could also be applied to relationship schemes other than parent-child hierarchies, such as network structures with other types of relationships. Taxonomies may then include single children with multi-parents, for example, “Car” might appear with both parents “Vehicle” and “Steel Mechanisms”; to some however, 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 seen as less broad than ontologies as 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 form the specific to the more general.