This report defines notional machines (NMs), and provides a series of definitional characteristics by which they may be identified. Over several sections, it includes a first-hand report of the origin of NMs, reports a systematic literature review to track the use and development of the concept, and presents a small collection of examples collected through interviews with experienced teachers. Additionally, the report presents NMs in a common format, and makes some preliminary explorations of their use in practice, including examples of instructors using multiple NMs in sequence. Approach and method are fully detailed in evidential appendices, to support replication of results and adoption/adaptation of practice.