- an abstract description of the system
- work done by developers with the title "architect"
- that diagram with clouds and arrows that somebody put on the network drive at the start of the project before we really knew what we were building
- that document signed by the client at the start of the project before we really knew what we were building
Martin Fowler's definition is more useful and widely applicable. He suggests that software architecture is the set of “things that people perceive as hard to change.” [PDF] This characterisation is successful because it shifts the focus from the production to the consumption of architecture.
Fowler's definition challenges the intentional fallacy as it applies to software architecture, which is the idea that the meaning of a text belongs to its author. Fowler's architecture can therefore include elements that were never deliberately envisaged by an architect, which in turn lets us consider systems that never had an architect.
A similar idea was advanced in the essay The Death of the Author by the literary critic Roland Barthes:
As soon as a fact is narrated no longer with a view to acting directly on reality but intransitively, that is to say, finally outside of any function other than that of the very practice of the symbol itself, this disconnection occurs, the voice loses its origin, the author enters into his own death, writing begins.Substitute "architect" for "author" and "development begins" for "writing begins" and Barthes could be talking about what happens when a carefully prepared architecture document is handed over for implementation.
Claude E. Shannon's information theory formally analyses the consumption of texts. He measured the information content of written English by showing test subjects a truncated piece of English text and asking them to guess what letter would come next. They guessed correctly about half the time (which means that English contains roughly 1 bit of information per letter).
The beauty of this experiment is that Shannon didn't need a model of his subjects' knowledge of English. All he had to do was observe was what happened when they applied that knowledge.
Implicit in Shannon's experiment is the idea that English is the sum of all cues that inform speakers as to what could come next. Following his approach, I would define architecture as the sum of all cues that suggest to a developer how a feature should be implemented in a particular system.
These cues can take many forms. Perhaps the arrows and clouds diagram tells a developer in which tier of the system to put a particular piece of logic. But developers are also guided by the language used by stakeholders, organisational structure and the culture of the technology stack.
Under this definition, the more prescriptive a system's architecture, the less information developers need to absorb in order to understand a given feature. In other words, architecture is the opposite of surprise.