Agnomen, n.
Etymology: [L.; ad + nomen name.]
1. An additional or fourth name given by the Romans, or account of some remarkable exploit or event; as, Publius Caius Scipio Africanus.
2. An additional name, or an epithet appended to a name; as, Aristides the Just.
Shades of meaning
Fernald has no entry of its own for agnomen; he lists it under name.
Where he writes about it:
Among the Romans the prenomen was the individual part of a man's name, the "nomen" designated the gens to which he belonged, the cognomen showed his family and was borne by all patricians, and the agnomen was added to refer to his achievements or character. — from name
Antonyms
none given