What is an Ontology?

In the knowledge engineering community is usually defined in the tradition of Gruber as an explicit specification of a conceptualization. Several variations of this definition exist. In the paper we argue that (with one notable exception) these definitions are of no explanatory value, because they violate one of the basic rules for good definitions: The defining statement (the definiens) should be clearer than the term that is defined (the definiendum). In the paper we propose a different definition of and discuss how it helps to explain various phenomena: the ability of ontologies to change, the role of the choice of vocabulary, the significance of annotations, the possibility of collaborative ontology development, and the relationship between ontological conceptualism and ontological realism.