Background: Adolescents' wellbeing is gaining increasing attention, with it being part of the United Nations sustainable goals (United Nations, 2019 [online]). The question regarding what adolescents' wellbeing is, however, has received no rigorous answer (Alexandrova, 2017), even though it is regarded as a fundamental measure of learning and growing as a human being (Stevens & Jarden, 2019). Understanding what wellbeing means to adolescents is vital and an under-researched area (Dunlop-Bennett et al., 2019) which has implications for the development and validation of tools to measure their wellbeing. Construct definition development literature highlights that experts, in addition to the relevant population, need to be consulted about the subject area to form a representative and valid definition (Podsakoff et al., 2016). Method: Two groups were invited to interview with the aim to understand their conceptualisations of wellbeing. Group 1 consisted of both expert practitioners and researchers from different disciplines in the field of wellbeing (n = 8) and Group 2 were young people aged 11-16 (n = 41). Conclusion: Any definition needs to be reflective of the multi-faceted nature of wellbeing. Findings revealed three themes: holism, positive feelings, and external influences. Based on these findings a definition of wellbeing was formed. These conclusions should be used to underpin research with adolescents and the development of a measure of young people’s wellbeing.