What is ‘wellbeing’? How important is ‘wellbeing’ to the field of positive psychology? I argue that the majority of the literature base of positive psychology rests on understanding ‘wellbeing’ and that accurately defining and conceptualising ‘wellbeing’ is an essential first, yet almost always completely overlooked or done poorly, step in the successful development of any wellbeing theory, policy, intervention, or research. This lack of clear definition and conceptualisation as to what ‘wellbeing’ is leaves positive psychology’s literature base on shaky foundations, and practitioners at risk of superficial understandings and suggestions. I will present research that indicates that distinctly different groups, such as adolescents, lawyers, teachers, Chinese students, or nurses, conceptualise psychological ‘wellbeing’ differently, and academics even more so. It is this lack of clear and agreed upon definition of ‘wellbeing’, or link back to cited literature, that is one of positive psychology’s biggest mistakes. As such, the aim of this talk is to synthesize findings to date, looking across different population groups, to provide insight into what 'wellbeing’ is and means. I highlight how these conceptualisations vary, and are different to academic notions of wellbeing. Given the scale and importance of this problem, I suggest implications for wellbeing theory development, current practice approaches, and advocate for a consensus definition of wellbeing – both for positive psychology and across the wellbeing sciences more broadly.