A quick definition of agnomen:

An agnomen is an extra name or title that someone might have, like a nickname. In ancient Rome, people had three names: a first name, a family name, and a nickname called a cognomen. Sometimes, people would get an agnomen to recognize something they did or because they were adopted into a different family. Nomen is another Latin word that means a personal name, and it was usually one of the three names that a Roman citizen had. Nomen can also mean any name in general. There are other Latin words related to names, like nomen collectivum, which means a name for a group of things, and nomen juris, which means a legal name or title.