This philosophical term (from metaphysics) refers to assertions or assumptions about the nature of reality: about what 'the real world' is like and what exists in it. It concerns what Foucault called 'the order of things' - a system of dividing up reality into discrete entities and substances. There are often hierarchical relations within an ontology: certain entities may be assigned prior existence, higher modality or some other privileged status. Ontologies are necessarily defined in words, which in itself transparently assigns a privileged status to words.