Rdf Primer / quote 31 of 55
A vocabulary is a set of URIrefs
Since RDF uses URIrefs instead of words to name things in statements, RDF refers to a set of URIrefs (particularly a set intended for a specific purpose) as a vocabulary. Often, the URIrefs in such vocabularies are organized so that they can be represented as a set of QNames using a common prefix. That is, a common namespace URIref will be chosen for all terms in a vocabulary, typically a URIref under the control of whoever is defining the vocabulary. [..] RDF uses this same approach to define its own vocabulary of terms with special meanings in RDF