A medical condition in which excess body fat has accumulated to the extent that it may have an adverse effect on health. Obesity is associated with diseases such as heart disease, type 2 diabetes, and certain types of cancer. The body mass index (BMI), which compares weight and height, is used to define a person as overweight (pre-obese: BMI = 25 kg/m2–30 kg/m2) and obese (BMI greater than 30 kg/m2).