cognomen

(noun) a familiar name for a person (often a shortened version of a person's given name). Synonyms: nickname, moniker, sobriquet, soubriquet, byname. Examples: "Joe's mother would not use his nickname and always called him joseph." "Henry's nickname was slim."

(noun) the name used to identify the members of a family (as distinguished from each member's given name). Synonyms: surname, family name, last name.