The cognomen (plural cognomina) formed one of the three parts of the typical Roman name. It was placed after the praenomen and nomen. Originally cognomina were nicknames, but by the time of the Roman Empire they were inherited from father to son. Thus the cognomen in combination with the nomen functioned as a surname, breaking families into smaller groups than just the nomen alone.

There were hundreds of cognomina in use. Some well-known examples are given in the following table.

Brutussenator Marcus Junius Brutus
Caesardictator Gaius Julius Caesar
Ciceroorator Marcus Tulius Cicero
Martialispoet Marcus Valerius Martialis (Martial)
Nervaemperor Marcus Cocceius Nerva
Scipiogeneral Publius Cornelius Scipio Africanus
Senecaorator Lucius Annaeus Seneca
Tacitushistorian Publius Cornelius Tacitus
Valerianusemperor Publius Licinius Valerianus (Valerian)