From Dick Buchanan’s 2001 essay Design, Making and the New Culture of Inquiry, describing the creation of the CMU graduate program in interaction design, here is the key passage focused on identifying this thing called “interaction design”:

Interaction design is the third great field of design to emerge in the 20th century. It combines qualities of visual communication and information design, which are characteristic of traditional graphic design, with the qualities of the whole body experience in a physical environment, which are characteristic of industrial design.

Interaction design is about people: how people relate to people, how people relate to products, and how people relate to each other through the mediating influence of products. It is a synthesis of many traditional and new elements of design thinking, organized into intelligent and emotionally satisfying experiences that meet a wide variety of human needs. Products are no longer treated simply as physical artifacts or visual symbols. Instead, they are expressions and enablers of human action and experience, situated in a social and cultural environment. For many of us, interaction design is more than a new branch of design practice. It is a new approach to design thinking in general, and a foundational critique of the entire field of design and the place of design in culture.

The idea of interaction design emerged in contemporary consciousness around problems of digital media and multimedia production, but it is by no means limited to the digital realm. Interaction design is equally important for traditional analog products as well as the new digital products that increasingly surround us.