A standard specifying the identifiers to be used for a particular collection of objects. Using standard vocabularies where they exist is key to enabling data integration. Linked data is rich in vocabularies in different topic areas. Source: ODH

A collection of terms for a particular purpose. Vocabularies can range from simple, such as the widely used RDF schema, FOAF and Dublin core metadata element set, to complex vocabularies with thousands of terms, such as those used in healthcare to describe symptoms, diseases and treatments. Vocabularies play a very important role in linked data, specifically to help with data integration. The use of this term overlaps with that of ‘ontology’.

Source: W3C — http://www.w3.org/TR/ld-glossary/#vocabulary