In the context of physical health, weight loss is the process of losing body weight, typically by losing fat. To achieve weight loss, more calories must be expended than taken in.

This can be achieved by reducing the amount of food consumed, increasing physical activity or a combination of the two. For healthy weight loss, most experts recommend a combination of healthy eating patterns and regular physical exercise. Other methods of losing weight include use of prescribed or herbal drugs or chemicals that decrease appetite and/or increase physical activity, such as fenfluoramine, ephedra, nicotine or cocaine (see anti-obesity drugs). Surgical options are also available. Liposuction removes targeted area of fat and tissue while bariatric surgeries can result in large weight loss by reducing the size of the stomach or reducing the absorption of food energy. Surgical and drug options may have serious side effects.

A loss of weight can also be the symptom of some mental or physical diseases or disorders, such as anorexia nervosa or the cachexia associated with cancer or AIDS.

Weight loss can help to 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.

Dieting is the practice or habit of eating (and drinking) in a regulated fashion, usually with the aim of losing weight. It is also used in some cases to gain weight or to regulate the amounts of certain nutrients entering the body. It usually involves a non-traditional diet.