Carolyn Snyder proposes a definition of paper prototyping as 'a variation of usability testing where representative users [...] [interact] with a paper version of the interface that is manipulated by a person "playing computer" ' (italicized in original, p.4).
The aim of her book is to extend the practice of paper prototyping to a variety of HCI platforms for non-expert users to adopt as a practical tool for creating and testing their products during the development process.