A system’s architecture is the shared understanding of this system’s purpose, context, function, form, structure and characteristics.
A system’s architecture results from planning, designing, building and sustaining this system.
In hindsight, mental model probably would have been a better term than definition.
The above was inspired by the thinking behind http://martinfowler.com/ieeeSoftware/whoNeedsArchitect.pdf and http://en.wikipedia.org/wiki/Architecture.
2014-04-26: Gene Hughson wrote about Architecture vs. Design at Iasa Global and Like it or not, you have an architecture (in fact, you may have several) at Form Follows Function.
2013-11-30: Added form, made function singular.
2013-11-09: Added characteristics to the definition. Stated mental model may be a better term than definition.