An agnomen is a fancy name that some really cool and important people in ancient Roman times used to have. It was like a nickname, but it sounded way fancier than "Bob" or "Sally".
So let's say there was a really important guy named Julius Caesar. He was so important, people admired and respected him so much that they gave him an agnomen. They called him "Caesarion," which meant "little Caesar." It's like how when your parents love you so much, they give you a cute nickname like "pumpkin" or "buttercup"!
Sometimes, people got agnomens based on things they did or special skills they had. For example, there was a guy named Publius Cornelius Scipio who was really good at fighting battles. Because he was so good at it, people gave him the agnomen "Africanus" because he had won a big battle in Africa.
So, an agnomen is basically a fancy nickname that important people got in ancient Rome to show how awesome they were!