What is a Model?

With the recent trend to model driven development a commonly agreed notion of “model” becomes a pivotal issue. However, currently there is little consensus about what exactly a model is and what it is not. Furthermore, basic terms such as “metamodel” are far from being understood in the same way by all members of the modeling community. This article attempts to start establishing a consensus about generally acceptable terminology. Its main contribution is the distinction between two fundamentally different kinds of models, i.e. “type model” versus “token model”. The recognition of the fundamental difference in these two kinds of models is crucial to avoid misunderstandings and unnecessary disputes among members of the modeling community.