Wellbeing is a growing area of research, yet the question of how it should be defined remains unanswered. This multi-disciplinary review explores past attempts to define wellbeing and provides an overview of the main theoretical perspectives, from the work of Aristotle to the present day. The article argues that many attempts at expressing its nature have focused purely on dimensions of wellbeing, rather than on definition. Among these theoretical perspectives, we highlight the pertinence of dynamic equilibrium theory of wellbeing (Headey & Wearing, 1989), the effect of life challenges on homeostasis (Cummins, 2010) and the lifespan model of development (Hendry & Kloep, 2002). Consequently, we conclude that it would be appropriate for a new definition of wellbeing to centre on a state of equilibrium or balance that can be affected by life events or challenges. The article closes by proposing this new definition, which we believe to be simple, universal in application, optimistic and a basis for measurement. This definition conveys the multi-faceted nature of wellbeing and can help individuals and policy makers move forward in their understanding of this popular term.