There's a pretty good discussion on the IxDA list regarding prototypes, what constitutes a prototype, what the goals are, etc. When I asked what tools people are using for prototyping, I received a long list of responses. One of the most interesting came from a respondent who didn't feel paper is a prototyping medium, but rather a design medium. Personally, I disagree with the notion of paper not being a prototyping tool—I've taught a number of workshops on it and use paper for prototyping in my work as a design researcher.

However, without actually defining what constitutes a prototype, it's rather difficult to say whether paper can be considered a prototyping tool or not. So, here's a few excerpts from my upcoming book that address the issue of defining a prototype:

[...] First, on an individual basis our definitions and descriptions of what constitutes a “prototype” vary greatly. Prototypes have been described as hi-fidelity, lo-fidelity, functional, nonfunctional, paper, Flash®, HTML, interactive, non-interactive, click-through, garden variety walk-through, cognitive walk-through—the list just keeps going.

Second, as a collective whole, prototypes share one thing—they are often incomplete. [...]

When I refer to a prototype, I'm referring to something that, from my book, "regardless of its fidelity, functionality, or how it’s made, a prototype is a conceptual model that captures and communicates the vision, intent, or idea of a design."

How would you define a prototype?