An implant is an artificial device made to replace and act as a missing biological structure. Outer shell of implants are often made of bioinert materials such as titanium. In some cases implants contain electronics e.g. artificial pacemaker and cochlear implants. In other cases medical implants has compound structure and acts as reinforcement e.g. dental implant or knee joint replacement implant. Some implants are active, as drug-eluting stents in the aorta and coronary arteries.

In Orthopedic surgery implants may refer to devices that are placed over or within bones to hold a fracture reduction and prosthesis would be the more appropriate term for devices that replace a part or whole of a defunct joint. In this context implants may be placed within the body (internal) or placed over the body (external).

An implant may also refer to a medication based on a drug suspended in a slow-release solid polypeptide carrier. These are usually administered by subcutaneous injection.