Design is a process of arriving to a finished, or as finished as time or energy allows, from an idea/thought/impression.
When applied to architecture, design is both a noun and a verb. You can design a building, and your building is a design. The operative notion is that design embodies the countless nights of thought, iteration and re-design, to approach a level of completeness that all parties can agree upon. In architecture this usually happens when the architect, client, building code inspector, and finally the construction industry can agree to a finished design.