Jus exclusivae

Jus exclusivæ (Latin for "right of exclusion"; sometimes called the papal veto) was the right claimed by several Catholic monarchs of Europe to veto a candidate for the papacy. The French monarch, the Spanish monarch, the Holy Roman Emperor, and the Emperor of Austria calimed this right at various times, making known to a papal conclave, through a crown-cardinal, that the monarch deemed a particular candidate for the papacy objectionable.

This right seems to have been claimed during the 17th century. It does not seem to be related to a right exercised by Byzantine emperors and Holy Roman Emperors to confirm the election of a Pope, which was last exercised in the Early Middle Ages. Spain, which ruled much of Italy at the time, raised the claim in 1605. In 1644, in the conclave which elected Cardinal Giovanni Battista Pamphili (who became Pope Innocent X), jus exclusivæ was first exercised, by Spain, to exclude Cardinal Sacchetti. Cardinal Jules Mazarin of France arrived too late at that conclave to present the veto of France against Cardinal Pamphili, who had already been elected. Around this period, treatises arise in defence of this right. At the 1846 conclave, Austrian Chancellor Klemens von Metternich confided Austria's veto of Cardinal Giovanni Maria Mastai-Ferretti (who became Pope Pius IX) to Cardinal Carlo Gaetano Gaisruck, Archbishop of Milan, who arrived too late.

...
Wikipedia