Originating from quite different fields of theory and practice, the terms "Enterprise Ontology" and "Enterprise Architecture" currently belong to the standard vocabulary of those professionals who are concerned with (re) designing and (re) engineering enterprises, thereby exploiting modern information and communication technologies for innovating products and services as well as for optimizing operational performance. Because of the inherent characteristics of modern enterprises, often operating within networks of cooperating enterprises, the task of these professionals can rightly be characterized as having to master unprecedented high complexity. The statement, put forward in the paper, that the current notion of Enterprise Ontology does not offer satisfactory help and thus needs to evolve into an effective conceptual tool, is clarified in a historical context. A standard is recently set by DEMO (Design and Engineering Methodology for Organizations). In this methodology, Enterprise Ontology is conceptually defined as the implementation independent essence of an enterprise, understood from a holistic systemic point of view. Operationally, it consists of a complete, consistent and coherent set of ontological aspect models, by which a reductio of complexity is achieved of well over 90%. The new, evolved notion of Enterprise Ontology is clarified and illustrated using a case example.