Polycephaly is the condition of having more than one head. The term is derived from the Greek stems poly meaning "many" and kephalē meaning "head". A polycephalic organism may be thought of as one being with a supernumerary body part, or as two or more beings with a shared body.

Supernumerary body parts are most commonly a congenital disorder involving the growth of an additional part of the body and a deviation from the body plan. Body parts may be easily visible or hidden away, such as internal organs.