The aforementioned Dick Gabriel posed a question to the Hillside Group: what is design? Here's my reply to him:
As a noun, design is the named (although sometimes unnamable) structure or behavior of an system whose presence resolves or contributes to the resolution of a force or forces on that system. A design thus represents one point in a potential decision space. A design may be singular (representing a leaf decision) or it may be collective (representing a set of other decisions).
As a verb, design is the activity of making such decisions. Given a large set of forces, a relatively malleable set of materials, and a large landscape upon which to play, the resulting decision space may be large and complex. As such, there is a science associated with design (empirical analysis can point us to optimal regions or exact points in this design space) as well as an art (within the degrees of freedom that range beyond an empirical decision; there are opportunities for elegance, beauty, simplicity, novelty, and cleverness).
A few related terms:
All architecture is design but not all design is architecture. Architecture represents the significant design decisions that shape a system, where significant is measured by cost of change.
In any given genre of systems, the same designs are found again and again. Insofar as we can name these designs and codify them in a relatively general fashion, they are the patterns of that domain.
Sometimes we will speak of a particular style (or school) of design. A style is a named collection of a set of designs that is observably identifiable from other styles.