An ontology is a formal description of the manner in which knowledge is represented and includes the "physics" of the world of that knowledge. As with monads, the term has longstanding philosophical and theological origins, but has a very specific and formal meaning in AI. Now, ontologies (some rather watered down) are becoming widespread and standard in web-based XML collaboration among federated systems. A useful ontology comprises a fact-based vocabulary, higher level facts about the world of those facts. There are ontological efforts completed or underway for many interesting domains, including business processes. PSL deals with manufacturing processes, and because of its superior formal underpinnings is a basis for ALF processes.

Lists of terms that provide formalized views of certain parts of the world. They explain entities within domains, their attributes and their relationships with other entities.