An ontology together with a set of individual instances of classes constitutes a knowledge base. In reality, there is a fine line where the ontology ends and the knowledge base begins. For the purposes of this study an ontology is a formal explicit description of concepts in a domain of discourse (classes, Sometimes called concepts), properties of each concept describing various features and attributes of the concept (slots, sometimes called roles or properties)), and restrictions on slots (facets, sometimes called role restrictions) (Gruber, 1993; Vlăduțescu, 2013).
International Letters of Social and Humanistic Sciences (Volume 41)
J. Novak-Marcincin et al., "The Standard of Axes in Ontology of Communication", International Letters of Social and Humanistic Sciences, Vol. 41, pp. 176-183, 2014