Research in psychopathology has identified psychological processes that are relevant across a range of Diagnostic and Statistical Manual (DSM) mental disorders, and these efforts have begun to produce treatment principles and protocols that can be applied transdiagnostically. However, review of recent work suggests that there has been great variability in conceptions of the term "transdiagnostic" in the treatment development literature. We believe that there is value in arriving at a common understanding of the term "transdiagnostic." The purpose of the current paper is to outline three principal ways in which the term "transdiagnostic" is currently used, to delineate treatment approaches that fall into these three categories, and to consider potential advantages and disadvantages of each approach.