Although mass customization, a term introduced by Davis (1987) to describe the oxymoron of mass producing customized products, has been part of research for more than a decade, literature has not come up with a commonly accepted definition of this term up to now. The present article attempts to close this gap by proposing a definition of traditional and electronic mass customization, which is based on answering three research questions. First, for which kind of customized goods (products versus services) is mass customization applicable at all? Second, at which step of the value creation process must the customer be given the chance to customize his or her good to be able to speak of mass customization? And finally, which prerequisites in terms of production cost and monetary price need to be fulfilled when comparing mass-customized with mass-produced goods? Using an extensive analysis of extant literature in the field, the authors develop two definitions of traditional mass customization, a working and a visionary one, as well as one for electronic mass customization, stating how new opportunities arising from advances in communication and information technology can influence this concept.