The body mass index or BMI (also known as the Quetelet index) measures the human body shape based on a person's mass (i.e. weight) and height. This is a simple way of measuring whether a person is within their ideal weight, or not (i.e. under- or overweight). There are a few shortcomings to this measurement, however, especially when applied to athletes who have higher muscle mass, who thus tend to weigh more than the average person.