What is Architecture?

Architect is defined by Architecture
Architecture is defined by architect.
Architect owns architecture and whatever Architect creates is Architecture. Read more here,

http://www.bredemeyer.com/whatis.htm

This gives some vague idea about Architecture. It defines it as a set of very high level technical decisions which are systemic and have a high impact. So scope and impact are the two dimensions on which the first quadrant will be the archiectural concerns.

Scope-Impact

S    |             |  Architectural concerns
C    |_____________|________________
O    |             |
P    |             |
E    |_____________|________________
                IMPACT

But there is one more dimension that some of the definitions revolve around - its the life of decisions. If the decisions live through the life of the product, most likely they will be architectural. This also means that these are decisons that are almost irreversible. Changing them is very difficult.

Architectural decisons are typically taken early in project lifecycle.

The above two points again tie into Scope-Impact model. Because of big scope and high impact the decisons are important enough to be taken early on. And the importance also guides the reversability.

Ofcourse each of these definition has exceptions. Once we have architecture, important thing is that Architect owns this. Own means - create, make changes and discard/replace at appropriate times.