The body mass index (BMI) or Quetelet index is a value derived from the mass (weight) and height of an individual. The BMI is defined as the body mass divided by the square of the body height, and is universally expressed in units of kg/m 2, resulting from mass in kilograms and height in metres.he modern term body mass index (BMI) for the ratio of human body weight to squared height owes its popularity to a paper published in the July 1972 edition of the Journal of Chronic Diseases by Ancel Keys.