OBJECTIVE: As highlighted in Australia's Productivity Commission Inquiry into mental health, subgroups of individuals are failing to have their needs met, or are 'falling through the cracks' in the current system - a phenomenon increasingly referred to as the 'missing middle'. A barrier to devising solutions is that the term 'missing middle' is not clearly defined. Using the Delphi method, we aimed to define the term and explore acceptability. METHOD: Three expert groups were recruited: carers and young people with a lived experience of mental ill-health; clinicians and service providers; researchers, policymakers and commissioners of mental health services. Using a three-stage Delphi process, we elicited definitions, refined and developed a consensus definition. RESULTS: Ten subthemes describing the 'missing middle' were identified, with four endorsed across all expert groups from the outset: service gap, inflexibility, inadequate service quality and duration, and social disadvantage. Additional subthemes were later endorsed. Feedback was sought on a consensus-driven definition that encompassed the original four endorsed subthemes. Findings supported a shift to a systemic focus - framing the 'missing middle' as a care gap. CONCLUSIONS: A consensus definition was developed, repositioning the term to a systems lens, describing a 'missing middle service gap'. The definition represents the 'missing middle' as a term to describe a gap in care where existing mental health services are not meeting the needs of individuals in a meaningful way. Research was carried out in relation to youth mental health in Australia and the definition may need to be adapted for other contexts.
