The term "menstrual health" has seen increased use across advocacy, programming, policy, and research, but has lacked a consistent, self-contained definition. As a rapidly growing field of research and practice a comprehensive definition is needed to (1) ensure menstrual health is prioritised as a unified objective in global health, development, national policy, and funding frameworks, (2) elucidate the breadth of menstrual health, even where different needs may be prioritised in different sectors, and (3) facilitate a shared vocabulary through which stakeholders can communicate across silos to share learning. To achieve these aims, we present a definition of menstrual health developed by the Terminology Action Group of the Global Menstrual Collective. We describe the definition development process, drawing on existing research and terminology, related definitions of health, and consultation with a broad set of stakeholders. Further, we provide elaboration, based on current evidence, to support interpretation of the definition.