"When a reasoning system adopts the open world assumption, it does not assume that its knowledge base is complete. That is there may be facts within its domain, that are true, but not in the knowledge base, This means it is not possible to say that something is false unless it is explicitly stated to be false in the knowledge base, RDF adopts an open world assumption.
The opposite is an closed world assumption."
Used on Chap. 17: pages 394, 395, 396
Also known as open world