The cognomen ("name known by" in English) was originally the third name of a Roman in the Roman naming convention. The term is also occasionally seen in modern times as an obscure synonym for nickname or epithet.
Because of the limited nature of Roman names, the cognomen developed to distinguish branches of the family from one another, and occasionally, to highlight an individual's achievement, typically in warfare. Some Romans notably general Gaius Marius had no cognomen at all. By the Late Roman Republic, however, the use of cognomen even in daily conversation had become typical.
In contrast to the honorary cognomen adopted by successful generals, most cognomen were based on a physical or personality quirk; for example, 'Rufus' meaning red-headed or 'Scaevola' meaning left-handed.
Today, we refer to many prominent ancient Romans by only their cognomen; for example, 'Cicero' serves as a shorthand for Marcus Tullius Cicero.