Only by applying the exclusively prescriptive notion of Enterprise Architecture as adopted in this book can one achieve organisational concinnity, which is one of the general goals of Enterprise Engineering. The current descriptive understanding of the notion in the practice of organisational design and ICT-applications development is imprecise and ambiguous. Several dominant definitions of architecture are reviewed and put aside as unsatisfying. The resulting prescriptive notion of Enterprise Architecture is the main vehicle to achieve organisational concinnity.