In medicine, a prosthesis, prosthetic, or prosthetic limb (from Ancient Greek prósthesis, "addition, application, attachment") is an artificial device extension that replaces a missing body part. It is part of the field of biomechatronics, the science of using mechanical devices with human muscle, skeleton, and nervous systems to assist or enhance motor control lost by trauma, disease, or defect. Prostheses are typically used to replace parts lost by injury (traumatic) or missing from birth (congenital) or to supplement defective body parts. Inside the body, artificial heart valves are in common use with artificial hearts and lungs seeing less common use but under active technology development. Other medical devices and aids that can be considered prosthetics include hearing aids, artificial eyes, palatal obturator, gastric bands, and dentures.

Prosthetics are specifically not orthotics, although given certain circumstances a prosthetic might end up performing some or all of the same functionary benefits as an orthotic. Prostheses are technically the complete finished item. For instance, a C-Leg knee alone is not a prosthesis, but only a prosthetic part. The complete prosthesis would consist of the stump attachment system - usually a "socket", and all the attachment hardware parts all the way down to and including the foot. Keep this in mind as often nomenclature is interchanged.