BMI stands for Body Mass Index, a widely used measurement in the fitness industry to assess body fat based on height and weight. It is calculated by dividing a person's weight in kilograms by the square of their height in meters. While BMI provides a useful screening tool for identifying potential weight-related health issues, it does not directly measure body fat or account for factors such as muscle mass, distribution, or specific individual variations, leading to critiques of its effectiveness for all populations.