In philosophy, ontology is the study of being. In knowledge-based systems, an ontology is that part of the system which specifies what things exist and what is true about them. Cyc’s ontology is the framework around which its knowledge base — consisting of axioms about the things that exist — is built. You may hear people refer to their “ontology of devices” or their “temporal ontology”. What they are talking about is those parts of their knowledge base (the constants and assertions) that concern devices or time.