Ontology in a philosophical context is study of the nature of being, existence or reality in general, as well as of the basic categories of being and their relations.

In an Information Technology context an Ontology is a model for describing the world that consists of a set of types, properties, and relationship types.

For the Semantic Web and Linked Data the models are based on the use of RDF, RDFS and OWL which are used to describe the types, properties, and relationship types.

Ontologies are also commonly known as RDF "vocabularies". However the word "vocabulary" is commonly used a number of different contexts, for example in the Drupal community the word "vocabulary" is often associated with the taxonomy module, not RDF. For clarity "ontology" is used here rather than vocabulary.

Finding Ontologies

Prefixes and Namespaces

Prefix.cc provides a lookup service for ontology namespace prefixes. The prefixes of commonly used ontologies are shown below.

Common Ontologies

Prefix Namespace Ontology
dc http://purl.org/dc/elements/1.1/ DCMI Namespace for the Dublin Core Metadata Element Set, Version 1.1
dcterms http://purl.org/dc/terms/ DCMI Namespace for metadata terms in the http://purl.org/dc/terms/ namespace
event http://purl.org/NET/c4dm/event.owl#
foaf http://xmlns.com/foaf/0.1/ Friend of a Friend (FOAF) vocabulary
geo http://www.w3.org/2003/01/geo/wgs84_pos# geo
gr http://purl.org/goodrelations/v1# GoodRelations describes product, price, and company data.
owl http://www.w3.org/2002/07/owl#
rdf http://www.w3.org/1999/02/22-rdf-syntax-ns# The RDF Vocabulary (RDF)
rdfs http://www.w3.org/2000/01/rdf-schema# The RDF Schema vocabulary (RDFS)
skos http://www.w3.org/2004/02/skos/core#
time http://www.w3.org/2006/time# Time
xsd http://www.w3.org/2001/XMLSchema#
void http://rdfs.org/ns/void# Vocabulary of Interlinked Datasets

Additional Ontology Listings

Ontology Design and Publication

Generating Ontology Documentation