A healthy body weight is one that makes you less likely to develop the various problems caused by excess body fat - especially some cancers, but also heart disease and diabetes, among others. Being overly thin or overly fat can be harmful, so you should aim to stay within the limits of healthy body weight. In other words, a healthy body weight is a weight (or BMI) that does not increase your risk of developing health problems or weight-related illnesses.
It is difficult to define an exact and healthy weight for each person because it would entail knowing the amount of lean (mostly muscle) tissue of each individual, which can only be done in a research lab.
