A meta-model is an explicit model of the constructs and rules needed to build specific models within a domain of interest. A valid meta-model is an ontology, but not all ontologies are mod-elled explicitly as meta-models. A meta-model can be viewed from three different perspectives:
1. as a set of building blocks and rules used to build models
2. as a model of a domain of interest, and
3. as an instance of another model.