Taxonomy is the practice and science of classification. The word comes from the Greek τάξις, taxis, ‘order’ + νόμος, nomos, ‘law’ or ’science’. Taxonomies, or taxonomic schemes, are composed of taxonomic units known as taxa (singular taxon), or kinds of things that are arranged frequently in a hierarchical structure, typically related 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. So, a thing needs to satisfy more constraints to be a car than to be a vehicle.
Applications
Originally the term taxonomy referred to the science of classifying living organisms (now known as alpha taxonomy); however, the term is now applied in a wider, more general sense and now may refer to a classification of things, as well as to the principles underlying such a classification.
Almost anything — animate objects, inanimate objects, places, concepts, events, properties, and relationships — may be classified according to some taxonomic scheme.
The term taxonomy may also apply to relationship schemes other than parent-child hierarchies, such as network structures with other types of relationships. Taxonomies may 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. So for instance, in common schemes of scientific classification of organisms, the root is called “Organism” followed by nodes for the ranks: Domain, Kingdom, Phylum, Class, etc.
