cognomen in WordNet English dictionary

1. a familiar name for a person (often a shortened version of a person's given name)
   "Joe's mother would not use his nickname and always called him Joseph"; "Henry's nickname was Slim"
   nickname  moniker  cognomen  sobriquet  soubriquet  byname 
   
2. the name used to identify the members of a family (as distinguished from each member's given name)
   surname  family name  cognomen  last name