There is disagreement about the origin of the term, but the consensus that "cardinalis" from the word cardo (meaning 'pivot' or 'hinge') was first used in late antiquity to designate a bishop or priest who was incorporated into a church for which he had not originally been ordained. In Rome the first persons to be called cardinals were the deacons of the seven regions of the city at the beginning of the 6th century, when the word began to mean "principal," "eminent," or "superior." The name was also given to the senior priest in each of the "title" churches (the parish churches) of Rome and to the bishops of the seven sees surrounding the city. By the 8th century the Roman cardinals constituted a privileged class among the Roman clergy. They took part in the administration of the church of Rome and in the papal liturgy. By decree of a synod of 769, only a cardinal was eligible to become bishop of Rome. Cardinals were granted the privilege of wearing the red hat by Pope Innocent IV in 1244.
In cities other than Rome, the name cardinal began to be applied to certain church men as a mark of honour. The earliest example of this occurs in a letter sent by Pope Zacharias in 747 to Pippin III (the Short), ruler of the Franks, in which Zacharias applied the title to the priests of Paris to distinguish them from country clergy. This meaning of the word spread rapidly, and from the 9th century various episcopal cities had a special class among the clergy known as cardinals. The use of the title was reserved for the cardinals of Rome in 1567 by Pius V.
In the year 1563 the influential Ecumenical Council of Trent, headed by Pope Pius IV, wrote about the importance of selecting good Cardinals. According to this historic council "nothing is more necessary to the Church of God than that the holy Roman pontiff apply that solicitude which by the duty of his office he owes the universal Church in a very special way by associating with himself as cardinals the most select persons only, and appoint to each church most eminently upright and competent shepherds; and this the more so, because our Lord Jesus Christ will require at his hands the blood of the sheep of Christ that perish through the evil government of shepherds who are negligent and forgetful of their office."
The earlier influence of temporal rulers, notably the French kings, reasserted itself through the influence of cardinals of certain nationalities or politically significant movements. Traditions even developed entitling certain monarchs, including those of Austria, Spain, and France, to nominate one of their trusted clerical subjects to be created cardinal, a so-called crown-cardinal.
In early modern times, cardinals often had important roles in secular affairs. In some cases, they took on powerful positions in government. In Henry VIII's England, his chief minister was Cardinal Wolsey. Cardinal Richelieu's power was so great that he was for many years effectively the ruler of France. Richelieu's successor was also a cardinal, Jules Mazarin. Guillaume Dubois and André-Hercule de Fleury complete the list of the four great cardinals to have ruled France. In Portugal, due to a succession crisis, one cardinal, Henry, King of Portugal, was crowned king, the only example of a cardinal-king.
While the incumbents of some sees are regularly made cardinals, and some countries are entitled to at least one cardinal by concordate (usually earning either its primate or the metropolitan of the capital city the cardinal's hat), no see carries an actual right to the cardinalate, not even if its bishop is a Patriarch.