Definition of obesity: Obesity is a medical condition in which excess body fat has accumulated to the extent that it can have an adverse effect on one's health. Obesity can be diagnosed using body mass index (BMI), a measurement of height and weight, as well as waist circumference. Obesity is categorized as having a BMI of 30 or greater. Abdominal obesity is defined as having a waist circumference greater than 40 inches for a man or 35 inches for a woman.