A Definition of Architecture

Brooks gives the following definition of software system architecture in his classic book The Mythical Man Month.

By the architecture of a system, I mean the complete and detailed specification of the user interface.

These days, user interfaces are collections of dropdowns, buttons, form fields, and styling choices. We'd hardly think of them as architectural.

But Brooks goes on to explain what he means by user interfaces, and by a detailed specification of as user interface:

For a computer this is the programming manual. For a compiler it is the language manual. For a control program it is the manuals for the language or languages used to invoke its functions. For the entire system it is the union of the manuals the user must consult to do his entire job.

We've largely given up on writing user manuals for software interfaces. User interfaces are supposed to be "intuitive" and "self-explanatory", using techniques like progressive disclosure. User interfaces don't come with documents. The modern equivalent of what Brooks is refering to is API documentation.

Then the modern interpretation of Brooks' definition of architecture is something along the lines of the collection of interfacts to the high-level application modules: the public APIs, object and methods that different parts of the system will consume to carry out tasks for the user.