I'm currently reading Software Systems Architecture: Working with Stakeholders using Viewpoints and Perspectives. I already think this is a book I should have read years ago! In chapter 2, the authors, Nick Rozanski and Eoin Woods break down the widely accepted definition of software architecture by the Software Engineering Institute (SEI) at Carnegie-Mellon University in Pittsburg:
The architecture of a software intensive system is the
structure or structures of the system, which compromise software elements, the externally visible properties of those elements, and the the relationships
I have seen this definition quoted many times but rarely examined in closer detail. Rozanski and Woods do however analyse structures and externally visible properties.
They state that there are two types of system structure, static and dynamic. Static structures of a software system define its internal design-time elements and their arrangement whilst dynamic structures of software system define it's runtime elements and their interactions.
Externally visible properties also manifest themselves in two ways, the externally visible behavior (what the system does) and quality properties (how the system does it). The externally visible behavior of a software system defines the functional interactions between the system and it's environment. A quality property is a nonfunctional property of a system such as performance, security, or scalability. These are commonly referred to as the "ilities".
So the key take away is that it's the role of the architect to derive the static and dynamic structures for each candidate architecture and understand the extent to which they exhibit the required behaviors and quality properties.