ABox

An ABox, or assertion box, is the component of a knowledge base in description logics and ontology languages that contains factual information about named individuals. In the usual separation, the TBox (terminology box) stores abstract schema such as class hierarchies and property definitions, while the ABox stores instance data; some frameworks also use an RBox for role-related axioms, but ABox is where concrete facts reside.

ABox assertions come in several forms. Concept assertions declare that a specific individual is an instance of a class, for example Person(Alice) meaning Alice is a Person. Role assertions declare relationships between individuals, for example hasParent(Alice, Bob). Data property assertions assign literal values to individuals, for example age(Alice, 30) or email(Alice, "[email protected]").

Reasoning over an ABox uses the accompanying TBox/RBox axioms to infer additional facts and to check consistency. Common tasks include instance checking (whether a given individual belongs to a class), consistency checking (whether the combined ABox and ontology is contradiction-free), and classification or realization (computing implicit class memberships for individuals). The open-world assumption typically applies, so absence of an assertion does not imply its negation.

In practice, ABoxes are used in ontology-based data access and semantic web contexts (for example OWL ontologies and RDF data) to store and query facts about individuals. They support integration of heterogeneous data sources by providing a structured set of instance-level facts that can be inferred and validated against the ontology's schema.