The definition of software architecture I use most often is Chris Verhoen's, which can be paraphrased as "that which is expensive to change." Those aspects of the system that are most expensive to change are also the ones that we most want to get right. In fact, I would argue that a software architect's value to a project is manifest whenever she avoids an expensive wrong decision that would have been made in her absence. It then follows that the role of a software architect is to avoid expensive mistakes. (Note, though, that inaction will also generally be a mistake.)
You might notice that this is a negative characterization of an architect's role: an architect's role is defined by what she does not do (make expensive mistakes), rather than by what she does. I'll come back to this later, after making another observation about the above definition of "architecture".
Defining architecture as "that which is expensive to change" provides an operational basis for determining if an aspect of the system is architectural. That is, given an aspect of the system, you can ask "how expensive would it be to change this?" to determine if that aspect is architectural: if it's expensive to change, then it's architectural; if it isn't, then it's not.
And this brings us to the title of this little essay, something I'll call the Paradox of Architecture. That is:
A large part of an architect's job is to avoid the introduction of architecture to a system.
I know this sounds completely backwards, and yet I believe it's true - the architect is responsible for preventing aspects of the design from becoming architectural. Consider the canonical example of a system designed without architecture, Foote and Yoder's Big Ball of Mud:
A BIG BALL OF MUD is haphazardly structured, sprawling, sloppy, duct-tape and bailing wire, spaghetti code jungle. We’ve all seen them. These systems show unmistakable signs of unregulated growth, and repeated, expedient repair. Information is shared promiscuously among distant elements of the system, often to the point where nearly all the important information becomes global or duplicated. The overall structure of the system may never have been well defined. If it was, it may have eroded beyond recognition.
A defining characteristic of these systems is that they are extremely difficult to maintain. Every aspect of the system is difficult (read: "expensive") to change. As such, every aspect of the system is architectural. Nothing is a simple "implementation detail", because no detail can be examined in isolation from the whole system. The whole system must be understood for every change. A Big Ball of Mud is nothing but architecture, and it is un-maintainable for that very reason.
On the other hand, consider the traditional Unix architecture. Doug McIlroy summarized the underlying philosophy beautifully as:
Write programs that do one thing and do it well. Write programs to work together. Write programs to handle text streams, because that is a universal interface.
Parts of the Unix architecture (like pipes, and untyped storage ("everything is a file")) are oriented around serving this philosophy. Nothing that did not serve this philosophy was included, leaving Unix with a limited set of primitive operations. The expressive power of this limited set is what makes us consider Unix to be a well-architected system. The rest of a Unix system (which comprises its bulk) is much easier to change, because the architectural components are small and isolated. Most of Unix's value is not in its architecture, which is what makes its architecture so good.