Ontology is a specification of a conceptualisation. That is, an ontology is a description of the concepts and their relationships. What is important is what an ontology is for. Ontological vocabulary (lexis) is designed for the purpose of enabling knowledge defining, sharing and reuse; it implies definitions of terms (words) and concepts (e.g. figures) for the purpose of constructing and understanding theories and hypotheses. [1,2]

[1] T Gruber  “What is an Ontology?”   http://www-ksl.stanford.edu/kst/what-is-an-ontology.html  (accessed 7 Novemb 2006)
[2]  T. R. Gruber. A translation approach to portable ontologies. Knowledge Acquisition, 5(2):199-220, 1993