The sobriquet Catholic King is a title awarded by the Pope as head of the Roman Catholic Church to a monarch who in the eyes of the papacy embodies Catholic principles in his or her personal live and state policies. The title remains attached to monarchs descended from whoever received the original sobriquet, unless withdrawn by a pope. The sobriquet can be awarded to either a king or a queen. One of the rights of being a Catholic King is that a queen may wear white, rather than the normal black, when meeting a pope.
See also Fidei defensor and Rex Christianissimus.