Population health is a relatively new term, with no agreement about whether it refers to a concept of health or a field of study of health determinants. There is debate, sometimes heated, about whether population health and public health are identical or different. Discussions of population health involve many terms, such as outcomes, disparities, determinants, and risk factors, which may be used imprecisely, particularly across different disciplines, such as medicine, epidemiology, economics, and sociology. Nonetheless, thinking and communicating clearly about population health concepts are essential for public and private policymakers to improve the population's health and reduce disparities. This article defines and discusses many of the terms and concepts characterizing this emerging field.