Definition of Body Mass Index(BMI): A measure of body weight relative to height. BMI can be used to determine if people are at a healthy weight, overweight, or obese. Since BMI uses total body weight (i.e., not estimates of fat and lean body mass separately) in the calculation, it does not discriminate between the overfat and the athletic or more muscular body type. Therefore, BMI should ideally be used in conjuntion with other body composition assessments.