Policymakers and others concerned about public health often speak of the need to achieve health equity. Yet the term can mean different things to different people. For government, other organizations, and communities, lack of shared understanding can be a serious obstacle to effective action. This lack of understanding makes it difficult to agree on concrete goals and criteria for success and can lead to wasted efforts, with policies and practices that work at cross-purposes. This article provides a carefully constructed definition of health equity and discusses the definition's implications both for action and for assessing progress toward health equity.