From W3C Wiki
The ontological closure of a web graph is formed by dereferencing URIs in the graph which occur in the predicate position in a triple (Properties), or in the object position when the predicate is rdf:type (Classes), adding the graphs returned by (DereferenceURI) and recursively dereferencing the URIs in them.
The OntologicalClosure is a form of WebClosure which in practice is finite and of reasonable size for a general RDF graph.
While one could design a web in which the ontological closure of a graph was huge, a reasonable convention would seem to be that one does not.
Many systems operate by taking the ontological closure of their input data.