The organizational structure of a system or component [IEEE Std 610.12-1990]
The fundamental organisation of a system embodied in its components, their relationships to each other, and to the environment, and the principles guiding its design and evolution. [IEEE Std 1471-2000]
The architecture of a software system is the structural and behavioural framework on which all other aspects of the system depend. It is the organisational structure of a software system including components, connections, constraints, and rationale.
A software system's architecture is the set of principal design decisions about the system. (Richard Taylor)