Rudi Studer, V. Richard Benjamins, and Dieter Fensel. Data & Knowledge Engineering25(1-2):161--197 (March 1998)Definition Ontology page 25: An ontology is a formal, explicit specification of a shared conceptualisation. A ‘conceptualisation’ refers to an abstract model of some phenomenon in the world by having identified the relevant concepts of that phenomenon. ‘Explicit’ means that the type of concepts used, and the constraints on their use are explicitly defined. For example, in medical domains, the concepts are diseases and symptoms, the relations between them are causal and a constraint is that a disease cannot cause itself. ‘Formal’ refers to the fact that the ontology should be machine readable, which excludes natural language. ‘Shared’ reflects the notion that an ontology captures consensual knowledge, that is, it is not private to some individual, but accepted by a group..