Wikipedia
A crown-cardinal was a cardinal protector of a Roman Catholic nation, nominated or funded by a Catholic monarch to serve as their representative within the College of Cardinals and, if applicable, exercise the jus exclusivae. More generally, the term may refer to any cardinal significant as a secular statesman or elevated at the request of a monarch.
Francis Burkle-Young defines a crown cardinal as one "elevated to the cardinalate solely on the recommendation of the European kings and without, in many cases, having performed any service at all for the advance of the Church."
According to conclave historian Frederic Baumgartner, the crown-cardinals "rarely came to Rome except for the conclaves, if then, and they were largely unknown to the majority of the College. Usually unable to take part in the pratiche, they were not papabili and rarely received more than one or two votes". Crown-cardinals generally opposed the election of crown-cardinals from other kingdoms, although they tended to unite against the election of cardinal-nephews.
Opposition to national cardinal protectors arose in the fifteenth century due to the perceived conflict of interest, and Pope Martin V attempted to forbid them entirely in 1425. A reform of Pope Pius II dated 1464 regards national cardinal protectors as generally inconsistent with curial responsibility, with several exceptions. Such protectorships were first openly permitted by popes Innocent VIII and Alexander VI, both of whom required the explicit written consent of the pontiff for a cardinal to take up a "position of service to a secular prince". An unnamed cardinal even suggested elevating national cardinal protectors to a full and official position in the Roman Curia, equivalent to an ambassador.