An ontology is a formal specification of a shared conceptualization. In the context of computer and information sciences, an ontology defines a set of representational primitives with which to model a domain of knowledge or discourse.

Ontologies are considered one of the pillars of the Semantic Web, although there exist many definitions.

Source: Gruer, T. What is an ontology?

ELISE Resources: Study on RDF & PIDs for INSPIRE: State-of-Play