In Computer Science, an ABox is an "assertion component"—a fact associated with a terminological vocabulary within a knowledge base.

The terms ABox and TBox are used to describe two different types of statements in ontologies. TBox statements describe a system in terms of controlled vocabularies, for example, a set of classes and properties. ABox are TBox-compliant statements about that vocabulary.

Tbox statements are sometimes associated with object-oriented classes and Abox statements associated with instances of those classes.

Together ABox and TBox statements make up a knowledge base.