Despite its undeniable currency in research and policy circles, there remains considerable confusion about what 'population health' is. We propose a lexicon for population health in the hope of clarifying issues and advancing this important research emphasis and policy agenda. It distinguishes population health in its literal meaning from a population health perspective, population health research, a population health framework, and a population health approach to policy. Population health is more than just thinking in aggregate terms or about identifying vulnerable or at-risk subpopulations. A population health perspective is fundamentally concerned with the social nature of health influences. The social structures that shape health experiences transcend the characteristics or actions of any one individual, providing population health with analytic advantages over individualistic-oriented approaches to health and to health policy.