byname (n.)

In ONOMASTICS, a supplementary name, added to someone’s personal name in order to help identification, and sometimes replacing it completely. For example, several singers with identical surnames in Wales are publicly known by their village of origin (e.g. Williams Penygroes, Williams Brynsiencyn). History is full of bynames – Eric the Red, James the Bold, Ethelred the Unready. A byname can in principle be distinguished from a surname, because it is not its purpose to be passed on between generations; however, many surnames undoubtedly started out life as bynames (e.g. Michael Carpenter).