From the mid‐1950's to the present the notion of a heuristic has played a crucial role in the AI researchers’ descriptions of their work. What has not been generally noticed is that different researchers have often applied the term to rather different aspects of their programs. Things that would be called a heuristic by one researcher would not be so called by others. This is because many heuristics embody a variety of different features, and the various researchers have emphasized different ones of these features as being essential to being a heuristic. This paper steps back from any particular research program and investigates the question of what things, historically, have been thought to be central to the notion of a heuristic and which ones conflict with others. After analyzing the previous definitions and examining current usage of the term, a synthesizing definition is provided. The hope is that with this broader account of ‘heuristic’ in hand, researchers can benefit more fully from the insights of others, even if those insights are couched in a somewhat alien voc