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

Derived forms: cognomens

Type of: appellation, appellative, denomination, designation, name

Encyclopedia: Cognomen