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The software architecture defines the coarse structures of the system and how to think about the software as a developer.
This is one of the more popular definitions:
The software architecture of a program or computing system is the structure or structures of the system, which comprise software elements, the externally visible properties of those elements, and the relationships among them.1)
There are plenty of other definitions, too.
Discuss this wiki article and the term on the corresponding talk page.