Straight from Dick Buchanan, as worded in his Interaction Design class syllabus:

“Design is an art of conceiving and planning products that are useful in supporting the activities of human beings in all areas of experience. Except in the area of crafts, designers do not make final products. They make prototypes and other visualizations, usually with supporting documentation that subsequently serve to guide manufacture and production.”

Continuing further about the nature of the profession of design…

“Design cannot be reduced to a fine art, engineering, or the social sciences without a significant loss in the richness, value, and utility of products. There may be strategic reasons for specialized work in one or another of these areas, but designers draw knowledge and insight from all of these areas in order to conceive and plan effective products.”

“Design is an integrative discipline, independent of specialized subject-matter disciplines. The vision behind this development is perhaps less evident than the visions that have reduced design successively to the fine arts, engineering, and the social sciences. But in the long term, the vision of design as an integrative discipline is more significant. Alliances between design and other disciplines will change from time to time as exciting new knowledge emerges in one or another field due to the contingent circumstances of research. But the essential advance of design–its ability to retain an identity and to incorporate new knowledge in the broader enterprise of design practice–will come from better understanding of the integrative nature of design thinking.”

And finally, the closer…

“Our department’s approach to design is fundamentally rhetorical in nature, in the sense that we regard design as a discipline that is based on the situatedness of products. This is a recognition that all products are situated in concrete, particular circumstances of human use, and that design must be a communicative art directed towards planning shaping human experience. The task of the designer is to conceive and plan products that are appropriate to human situations, drawing whatever knowledge and ideas are needed from all of the arts and sciences. For this reason we have identified communication and the human experience in design as the fundamental theme of the department.”

And one more thing :-)

“Design is an art of practical deliberation oriented towards shaping the argument of all products. The core of design, therefore, is an argument that integrates logos, mythos, ethos, and pathos. This argument is not expressed in words…they are vividly embodied in images, objects, actions/services, and systems.”