The term ontology is used by philosophers to describe that branch of metaphysics that deals with the question of `What things exist?' In recent years `ontology' has become a buzz word in information science, where it refers to a rigorous formal specification of a vocabulary of concepts and relations. Such specifications are playing an increasingly important role in ensuring the integrity of data both within single, information-rich applications and in the transfer of data between applications.
The Ontobase consists of a set of Prolog files which store formula sets together with certain important meta-information describing the status of each formula, the properties and intended interpretation of theories and concepts and relationships between the various sub-theories. The theory web pages are generated automatically from the prolog source files (you can look at the source via a link on each theory page).
The database has a hierarchical structure in which one theory can incorporate others as sub-theories. Where a theory is incorporated in this way, its domain of quantification may be restricted to coincide with the extension of a specified predicate of the larger theory in which it is included.