Weight loss, in the context of medicine or health or physical fitness, is a reduction of the total body weight, due to a mean loss of fluid, body fat or adipose tissue and/or lean mass, namely bone mineral deposits, muscle, tendon and other connective tissue.

Weight loss may refer to the loss of total body mass in an effort to improve fitness, health, and/or appearance.

Therapeutic weight loss, in individuals who are overweight, can decrease the likelihood of developing diseases such as diabetes. Overweight and obese individuals face a greater risk of health conditions such as type 2 diabetes, heart disease, high blood pressure, stroke, osteoarthritis and certain types of cancer. For healthy weight loss, a physician should be consulted to develop a weight loss plan that is tailored to the individual.

Weight loss occurs when an individual is in a state of negative energy balance. When the human body is spending more energy in work and heat than it is gaining from food or other nutritional supplements, it will catabolise stored reserves of fat or muscle.

Although weight loss may involve loss of fat, muscle or fluid, weight loss for the purposes of maintaining health should aim to lose fat while conserving muscle and fluid.