BMI, or Body Mass Index, is a numerical value calculated from a person's height and weight, used as a general indicator of body fatness and health risk. It is calculated by dividing a person's weight in kilograms by the square of their height in metres.

While BMI is a widely used screening tool, it has limitations — it does not account for factors such as muscle mass, age, sex, or fat distribution, meaning it may not accurately reflect the health of athletes, the elderly, or certain ethnicities. It is best used as a starting point rather than a definitive measure of health.