Name of a legendary Eastern priest and king.
The fabulous wealth of this head of a supposed Christian kingdom in the Far East furnished abundant material for writers of the Middle Ages, e.g., Sir John Mandeville, now considered unreliable, and Wolfram von Eschenbach (in "Parsifal").
According to Marco Polo, Prester John was Unc-Khan, and for centuries that Prince of the Keriats, a Mongolian tribe, was believed to be Prester John of the legend; his sacerdotal character was considered due to the fact that he might have been dedicated to the priesthood in his cradle according to the Nestorian custom.
In Jerusalem, early in the 15th century, the Abyssinian priests described their country to the Portuguese merchants as the Kingdom of Prester John, which accounts for the persistent search, by the Portuguese discoverers of that century, for the kingdom and for the king himself, along the coast of Africa and the East Indies.
New Catholic Dictionary