BMI, or Body Mass Index, is a ratio that is calculated using a person's weight and height. It is defined mathematically as:
\[ \text{BMI} = \frac{\text{Weight (kg)}}{\text{Height (m)}^2} \]
This gives a numerical value that is used to categorize individuals into various weight status categories, such as underweight, normal weight, overweight, and obese. It is a commonly used tool to assess whether a person has a healthy body weight for their height.