Research and action to address social disparities in cancer requires clarity about what constitutes and causes these persistent and onerous inequities in health. Currently, both scientific literature and government documents exhibit important disagreements, confused terminology, and considerable, if not deliberate, vagueness about the meaning of the phrase 'cancer disparities' and the related term 'social disparities in health.' This article accordingly reviews critical issues relevant to cohering understanding of what is meant by 'cancer disparities;' offers a definition premised on the causal contention that social disparities in health, by definition, arise from social inequity; and considers its implications for developing a multidisciplinary research agenda on social inequalities in cancer. Tackling this issue will require rigorous and critical frameworks, questions, and methods derived from multiple disciplines, and will necessarily involve epidemiologic, clinical, and intervention research, both quantitative and qualitative. At issue is making conscious research choices: about which types of disparities we study, in relation to which aspect of cancer, so as to improve the likelihood our research will help inform a society-wide discourse about the extent, origins of, and remedies for social injustices in cancer, thereby aiding efforts to eliminate social inequalities in health.