Some classic definitions: http://www.sei.cmu.edu/architecture/definitions.html
- …architecture is concerned with the selection of architectural elements, their interactions, and the constraints on those elements and the interactions necessary to provide a framework in which to satisfy the requirements and serve as a basis for the design [Perry and Wolf].
An architecture is the set of significant decisions about the organization of a software system, the selection of the structural elements and their interfaces by which the system is composed, together with their behavior as specified in the collaborations among those elements, the composition of these structural and behavioral elements into progressively larger subsystems, and the architectural style that guides this organization---these elements and their interfaces, their collaborations, and their composition [Booch, Rumbaugh, and Jacobson, 1999]