Vocabularies in the VO Version 2.1

In this document, we discuss practices related to the use of RDF-based consensus vocabularies in the Virtual Observatory, that is the creation, publication, maintenance, and consumption of hierarchical word lists agreed upon within the IVOA. To cover the wide range of use cases envisoned, we define different vocabulary types for informal knowledge organisation on the one hand, and strict hierarchies of classes and properties on the other. While the framework rests on the solid foundations of W3C RDF, provisions are made to facilitate using IVOA vocabularies without specific RDF tooling. Non-normative appendices detail the current vocabulary-related tooling.