The question of whether obesity should be regarded as a disease remains controversial. One source of controversy can be addressed by distinguishing between two ways in which the word “obesity” is used. In medicine, the word “obesity” now typically refers to some or all of a set of interrelated dysfunctions of metabolism, adipose tissue, and dietary intake regulation. In other contexts, such as government-funded public education programs, the word “obesity” refers to a body mass index (BMI) category taken to indicate excess body fat. The result is that when medical experts say, “Obesity is a disease,” the majority of outside medicine inevitably takes this to mean “being fat is a disease.” In order to address this ambiguity, we apply key philosophical accounts of disease to the two different senses of “obesity.” We draw two major conclusions: First, although obesity as understood in clinical medicine meets the criteria to be considered a disease, obesity as defined by BMI does not. Second, adequately addressing this disease requires us to distinguish it clearly and unambiguously from high BMI. Making this distinction would help both the public and policymakers to better understand the disease of obesity, facilitating advances in both prevention and treatment.