1.NounA personal epithet or clan name added to the given name and family name of Ancient Romans.
"Julius Caesar's actual name was Gaius Iulius Caesar. Gaius was his praenomen or forename, Iulius his nomen or surname, and Caesar his cognomen, denoting which part of the Iulius family he belonged to."
2.NounSynonym of nickname, any epithet used similar to the Roman cognomina.