At a grossly oversimplified level, the Semantic Web consists of semantic islands and semantic links between those islands, each represented by a uniform resource identifier (URI) and a collection (graph) of very brief statements (triples) in turn constructed from URIs. In essence, each of these semantic islands is a concentration of knowledge, as is each of the semantic links as well.
But what about all of the space between islands as well as all of the dotted-line links that are too weak to be recognized as formal semantic links? All of this white space that is somehow not a recognized part of the semantic map of formal knowledge. I'll refer to all of this informal knowledge that is outside and beside and under and over and around and in between the formal islands of knowledge and the formal links between these formal islands of knowledge as semantic white space.
Semantic white space is informal knowledge that exists in the margins of our formal knowledge which is just as important, but usually is accorded even less status than a second-class citizen in the hierarchy of knowledge.
If we are ever to construct a true consumer-centric knowledge web we will need to accord informal knowledge and semantic white space first-class status on a par with formal knowledge.