Ontology in Knowledge Engineering is a conceptualization of a domain defined in a formal language understandable both by humans and computers (“an ontology is an explicit specification of a conceptualization.” T. Gruber, 1993).
« An ontology is a conceptualisation of a domain to which one or several vocabularies can be associated and which participates to the meaning of terms. Defined for a given objective, an ontology expresses a point of view shared by a community. An ontology is represented in a language (explicit ontology) whose theory (semantics) guarantees the properties of the ontology in terms of consensus, coherence, sharing and reuse. » 
Shared by a community of practice, an ontology specifies a consensual knowledge ground for defining terms in a specialized domain (a term is a “verbal designation of a concept” [ISO 1087-1: “Terminology work – Vocabulary – Part 1: Theory and application”]).
Combining terminology and ontology leads to the notion of ontoterminology, a terminology whose conceptual system is a formal ontology. Ontoterminology opens up new axes of research and finds many applications in domains such as the semantic web, digital humanities and the industrial world. 
 Ontology: a survey. C.Roche. 8th Symposium on Automated Systems Based on Human Skill and Knowledge – IFAC, September 22-24 2003, Göteborg, Sweden
 Ontoterminology: How to unify terminology and ontology into a single paradigm. C.Roche. LREC 2012, Eighth international conference on Language Resources and Evaluation, Istanbul (Turkey), 21-27 May 2012, pp. 2626-2630