Eliminating health disparities is a Healthy People goal. Given the diverse and sometimes broad definitions of health disparities commonly used, a subcommittee convened by the Secretary's Advisory Committee for Healthy People 2020 proposed an operational definition for use in developing objectives and targets, determining resource allocation priorities, and assessing progress. Based on that subcommittee's work, we propose that health disparities are systematic, plausibly avoidable health differences adversely affecting socially disadvantaged groups; they may reflect social disadvantage, but causality need not be established. This definition, grounded in ethical and human rights principles, focuses on the subset of health differences reflecting social injustice, distinguishing health disparities from other health differences also warranting concerted attention, and from health differences in general. We explain the definition, its underlying concepts, the challenges it addresses, and the rationale for applying it to United States public health policy.