Agnomen

An agnomen (Latin: [aŋˈnoːmɛn]; pl.: agnomina), in the Roman naming convention, was a nickname, just as the cognomen had been initially. However, the cognomina eventually became family names, and so agnomina were needed to distinguish between similarly-named persons. However, as the agnomen was an additional and optional component in a Roman name, not all Romans had an agnomen.

Pseudo-Probus uses the hero of the Punic Wars, Publius Cornelius Scipio Africanus, as an example:

Men's personal names are of four types, praenomen, nomen, cognomen and agnomen: 'praenomen for instance Publius, nomen Cornelius, cognomen Scipio and agnomen Africanus.

Marius Victorinus further elucidates:

Now the agnomen comes from outside, and in three styles, from personality or physique or achievements: From personality, such as Superbus ["Haughty"] and Pius [displaying the Roman syndrome of virtues including honesty, reverence to the gods, devotion to family and state, etc.], from physique, such as Crassus ["Fatty"] and Pulcher ["Handsome"], or from achievements, such as Africanus and Creticus [from their victories in Africa and on Crete].

Africanus, Creticus and the likes are also known as victory titles. For example, Gaius Marcius Coriolanus earned his from the capture of Corioli.