An ontology or vocabulary is a set of concepts and categories in a subject area or domain that shows their properties and the relations between them.
‘There is no clear division between what is referred to as “vocabularies” and “ontologies”. The trend is to use the word “ontology” for more complex, and possibly quite formal collection of terms, whereas “vocabulary” is used when such strict formalism is not necessarily used or only in a very loose sense. Vocabularies are the basic building blocks for inference techniques on the Semantic Web.’1
- Glossary page on Semantic Web
- Glossary page on OWL 2 Web Ontology Language