Researchers have used a myriad of different definitions in seeking to explain the heterogeneity of family firms and their unique behavior; however, no widely-accepted definition exists today. Definitional clarity in any field is essential to provide (a) the basis for the analysis of performance both spatially and temporally and (b) the foundation upon which theories, frameworks and models are developed. We provide a comprehensive analysis of prior research and identify and classify 82 definitions of family business. We then review and evaluate five key theoretical perspectives in family business to identify how these have shaped and informed the definitions employed in the field and duly explain family firm heterogeneity. Finally, we provide a conceptual diagram to inform the choice of definition in different research settings.