Despite the expanded focus on "population health" since passage of the Affordable Care Act, the term is not formally defined in the law. Adding to the confusion, the Centers for Medicare and Medicaid Services uses the term to refer to the health of beneficiaries assigned to an accountable care organization (ACO), rather than the health of people living in a defined geographical area. For their Commonwealth Fund–supported study, researchers interviewed leaders at ACOs and public health agencies to find out how they define "population health," "public health," and "community health."