Obesity is used to describe an individual weight or accumulation of body fat that is considered unhealthy. The most common measure to determine obesity is the body mass index (BMI), although this measure is inherently limited in its accuracy. Specifically, an adult with a BMI of 30 or higher is considered obese. An adult with a BMI between 25 and 29.9 is considered overweight. The terms obesity and overweight in this context refer specifically to body fat and not muscle or other weight.

Obesity, weighing too much for a person's frame, age, and sex, is linked to a number of negative health conditions including heart disease, musculoskeletal disorders, cancer and diabetes. Current guidlines consider a person with a BMI of 30 or higher to be obese. BMI is a calculation of an individual’s ratio of height to weight. BMI does not measure body fat directly4 but is used as a screening tool. Because BMI is not a diagnostic tool, it cannot measure the healthiness of a particular individual. Skinfold thickness measurements, bioelectrical impedance, densitometry (underwater weighing), dual energy x-ray absorptiometry (DXA) and other testing may be used to clinically determine the actual body fat of a specific individual. However, BMI measurements have been shown to be a strong indicator of overall body fat content level.