TBox

TBox, short for terminological box, is a component of knowledge bases used in description logic and ontology engineering. It stores the terminological knowledge: definitions, relationships, and constraints that describe how concepts and roles are organized. The TBox captures the vocabulary and its structure, such as class hierarchies and property relationships, while the corresponding ABox contains assertions about individual instances.

In practical terms, the TBox includes statements about which classes exist, how they relate to one another, and how properties apply to those classes. Common elements are subclass relations (one class is a subset of another), equivalence between classes, disjointness of classes, and constraints on properties such as domain and range. The TBox may also specify characteristics of properties, such as transitivity or symmetry, and can include complex concept definitions built from existing ones.

In ontology languages like OWL, the TBox aligns with the schema or ontology portion, defining the vocabulary and its structure. The ABox, by contrast, contains instance-level facts about specific individuals. Reasoners use the TBox to infer new knowledge when combined with the ABox, enabling automatic classification and consistency checking.

Examples of TBox content include statements such as: Dog is a subclass of Mammal; the property hasParent has domain Person and range Person; and the classes Bird and Fish are disjoint. TBox is fundamental for semantic interoperability and knowledge reuse in information systems and the semantic web. See also ABox and Description Logic.