The substitution of a title or epithet for a proper name, as in calling a sovereign "Your Majesty.".
The substitution of a personal name for a common noun to designate a member of a group or class, as in calling a traitor a "Benedict Arnold".
Use of a proper name to suggest its most obvious quality or aspect.
In rhetoric, the substitution of an epithet, or of the appellative of some office, dignity, profession, science, or trade, for the true name of a person, as when his majesty is used for a king, his lordship for a nobleman, or the philosopher for Aristotle; conversely, the use of a proper noun in the place of a common noun: as, a Cato for a man of severe gravity, or a Solomon for a wise man.
He blithely absolves this libel as an example of "antonomasia" ( "the use of a proper name to express a general idea," OED), saying rather too airily that in this instance he meant a "Pat Boone-type" to imply any crooner of the well-scrubbed variety.
Garibaldi was always or almost always victorious (in reality he fought brilliant guerrilla skirmishes which piety later turned into vast and tidy battles); he was the first to be called Il Duce, a pompous nineteenth-century opera libretto title, by antonomasia (Mussolini had been called Il Duce by his socialist followers before 1914 and took the title with him to the Fascist party).
In Florence, a rich and famous city of Italy, in the province called Tuscany, there dwelt two rich and principal gentlemen called Anselmo and Lothario, which two were so great friends, as they were named for excellency, and by antonomasia, by all those that knew them, the Two Friends.
The word 'antonomasia' comes from a Greek word meaning 'to name instead'.