What term describes an additional name gained through adoption or achievement in ancient Rome?

The term that describes an additional name gained through adoption or achievement in ancient Rome is "agnomen." In Roman naming conventions, a person typically had three names: the praenomen (given name), the nomen (the clan or family name), and the cognomen (a nickname or additional family designation). The agnomen was an extra name that was often conferred to honor someone's accomplishments or to indicate a particular circumstance, such as a notable achievement in battle or a unique lineage through adoption.

For instance, a famous example of an agnomen is "Africanus," which was added to the name Scipio to indicate his victory over Hannibal in the Second Punic War. This usage highlights the significance of the agnomen in Roman society as a marker of identity and status.

The other terms mentioned refer to different aspects of Roman names. The nomen relates to the family or gens a person belongs to, while the cognomen indicates a branch of that family or serves as a nickname. The word "titulo" does not pertain to Roman naming conventions at all.