The definition of BMI is the Body Mass Index, which measures body fat based on height and weight. It is calculated by dividing weight in kilograms by the square of height in meters. The significance of BMI lies in its role as a screening tool for categorizing individuals into different weight classes, such as underweight, normal weight, overweight, and obese. Although useful for assessing general health risks associated with weight, BMI does not account for variations in muscle mass or fat distribution, which may lead to misinterpretations in certain populations.