The discipline model-based systems engineering (MBSE) has become very famous in systems engineering in the last years. INCOSE defines MBSE as “the formalized application of modeling to support system requirements, design, analysis, verification, and validation activities beginning in the conceptual design phase and continuing throughout development and later life cycle phases” . The most recent INCOSE Vision 2025  mentions system architecture as a crucial discipline for future successful systems engineering. Model-based system architecture combines both enablers.
Although the term model is essential, there is no common definition for the term model in the context of MBSE. Stachowiak defines in his book about general model theory three features of a model :
The mapping and reduction features imply another often-mentioned feature of models: abstraction. Abstraction is the process to reduce the information about a concept to the relevant parts for a particular purpose. An abstraction is the result of an abstraction process. See also Section 11.2.5 about the abstraction skill of system architects.
We second the features of Stachowiak and add some more to give a definition how we understand and use the term model respectively ...