"cognomen" in the noun sense
1. nickname, moniker, cognomen, sobriquet, soubriquet, byname
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"
2. surname, family name, cognomen, last name
the name used to identify the members of a family (as distinguished from each member's given name)
Source: WordNet® (An amazing lexical database of English)
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