Weight loss, in the context of medicine or health, is a reduction of the total body weight, typically due to catabolism of stored reserves inside the body. Limited short-term weight loss may be caused by the loss of water from the body; however, the typical reason for weight loss is that the human body is losing more chemical energy in work and heat than it is gaining from food. The body then can use its stored reserves of chemical energy, such as fat, to make up the deficit. The byproducts of using these reserves then leave the body, reducing body weight. This process may be triggered by the reduction of the Caloric content of the diet, an increase in physical activity, the immune system fighting off a disease, or other causes. Weight loss is a common symptom in medicine.

In a less formal sense, weight loss may refer to the loss of body fat in an effort to improve fitness, health, and/or appearance. This form of weight loss can be achieved by a combination of healthy eating patterns and regular physical exercise. Other methods of losing weight include use of drugs that decrease appetite, as well as surgery. Liposuction is the surgical removal of fat from targeted areas beneath the skin. Bariatric surgery artificially reduces the size of the stomach, reducing the intake of food energy. Some of these treatments may have serious side-effects.

Therapeutic weight loss can help relieve symptoms of a variety of conditions, such as diabetes, back pain and disorders of the spine, pain in the knee or legs, and many heart conditions. For healthy weight loss, a physician should be consulted to develop a weight loss plan that is tailored to the individual.