Source : Webster's Revised Unabridged Dictionary (1913)

Agnomen \Ag*no"men\ ([a^]g*n[=o]"m[e^]n), n. [L.; ad + nomen
   name.]
   1. An additional or fourth name given by the Romans, on
      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.