An extensional definition of interdisciplinarity--that is, a definition that identifies the types of practices that are interdisciplinary--is explained, advocated, and developed. The criteria for identifying appropriate interdisciplinary practices are outlined. Such a definition is useful both for interdisciplinary teachers and researchers who reflect on the nature of interdisciplinary practice, and for students and administrators who wish to better appreciate what interdisciplinarity involves. Critically, it facilitates identification of where along a continuum from interdisciplinarity to disciplinarity a particular field, course, or project lies. Some scholars or fields might then decide, and--importantly--know how, to become more interdisciplinary. Such a definition is arguably necessary, though hardly sufficient, in our efforts to advocate for better administrative treatment of interdisciplinarity.