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.
Prefixes and Namespaces
Prefix.cc provides a lookup service for ontology namespace prefixes. The prefixes of commonly used ontologies are shown below.
Additional Ontology Listings
Ontology Design and Publication
Generating Ontology Documentation