Though the above noders are correct in their definition of Body Mass Index, they are incorrect about its applicability.
Body Mass Index is NOT an accurate measure of a person's "fatness".
First, a bit of history. The Body Mass Index scale is the brainchild of a Belgian mathemetician named Lambert Adolphe Jacques Quetelet. He was NOT a physician. Mr. Quetelet first released this formula in the 1800's as a quick way to measure the relative fatness of the population and thus allow the government to allocate resources appropriately. The formula is based off of Quentelet's idea of an "average man". Although averages are fine when used to measure populations, measuring against an average is seldom a good idea for individuals. In fact, Quetelet himself even said BMI could not and should not be used to measure the level of fatness of an individual. Body Mass Index is a fundamentally flawed system of measure and has no place in modern medicine. Our continued reliance on such an inadequate measuring system is detrimental to the health of our society, as well as our economy.
BMI is anatomically and logically wrong. Physiologically speaking, there is no reason to divide someone's weight over the square of their height. Quetelet essentially reverse engineered this formula; thus if a person is fat, they will have a high BMI. However, having a high BMI definitely does not mean a person is fat. This is because BMI only accounts for height and weight. It does not account for several other variables involved in calculating "fatness", most notably bone density and muscle mass, i.e people that have strong bones and well toned muscles will be classified as overweight or obese, though they may not be. Relative shoulder width and body girth are two of several other factors that should be taken into account when measuring the fatness of an individual.
Most importantly, the Body Mass Index scale creates distinct definitions of "Underweight", "Overweight", and "Obese". This is complete garbage, because every individual is just that, an INDIVIDUAL! You can't judge if someone is underweight, overweight or obese based on the weight of the "average" person of their height. Just because someone is not "average" doesn't mean they are unhealthy, fat, or even underweight!
A continued reliance on such a flawed system to measure the fatness of individual people is undoubtedly wrong. People rely on their doctors to keep them healthy; if the doctors are in turn relying on an inaccurate tool, they are being unfair to their patients. The harmful effects of using such a scale are numerous. First off, the mental health of the population may be adversely affected. Our society already hears enough about how we don't look exactly like the cracked-out runway models and anorexic celebrities we see on TV every day.
Secondly, because doctors are constantly telling their patients they're overweight, fad diets have become a billion dollar industry. People are subjecting themselves to the potential kidney damage caused by "the Atkins diet" and other no-carb diets, not to mention the countless other severe deficiencies and problems with fad diets and weight loss products, all in the name of caring for their "health". Is this really health we are chasing, or acceptance?
Finally, health insurance companies (at least here in America) have the right to increase their premiums for people with high BMI's. Regular, healthy individuals are being charged more, even in our jobless economy, just because they have strong bones or toned muscles. Am I the only one who thinks this is ridiculous?
It's high time modern medicine introduced a new, more accurate system of measuring obesity, because this one is clearly broken.