cognomen

The cognomen (/kɒɡˈnoʊmɛn/, /ˈkɒɡnəmən/; Latin: [koːŋˈnoːmen]; Latin plural cōgnōmina; con- "together with" and (g)nōmen "name") was the third name of a citizen of ancient Rome, under Roman naming conventions. The cognomen started as a nickname, but lost that purpose when it became hereditary. Hereditary cognomina were used to augment the second name (the family name, or clan name) in order to identify a particular branch within a family or family within a clan. Outside of this particular use of the word, the term has taken on a variety of other meanings in the contemporary era.