An agnomen (plural: agnomina), in the Roman naming convention, is a nickname, much like how cognomen was initially. However, the cognomina eventually became family names, so agnomina was 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 a agonomen (at least not recorded). Pseudo-Probus uses the hero of the Punic Wars, Publius Cornelius Scipio Africanus, as an example: Marius Victorinus further elucidates: Africanus, Creticus and the likes are also known as victory titles. For example, Coriolanus earned his from the capture of Corioli.
As a minimum, a Roman agnomen is a name attached to an individual's full titulature after birth and formal naming by the family. True Roman nicknames, fully replacing the individual's name in usage, are rare. An example is Caligula, which was used in place of, and not along with, his full name, which was Gaius Iulius Caesar Augustus Germanicus. Caligula's praenomen was Gaius, his nomen Iulius, his cognomen Caesar. Some agnomina were inherited like the cognomen, thus establishing a sub-family.
An agnomen is not a pseudonym, but a real name; agnomina are additions to, not substitutions for, an individual's full name. Parallel examples of agnomina from later times are epithets like Thomas Jonathan "Stonewall" Jackson (though he is known more often by his agnomen than his Christian name) or popular nicknames like "Iron" Mike Tyson.
agnomen in Czech: Agnomen
agnomen in Italian: Convenzione dei nomi romani#Agnomen
agnomen in Dutch: Agnomen
agnomen in Turkish: Agnomen