A cardinal is a senior ecclesiastical official, usually a bishop, of the Catholic Church. They are collectively known as the College of Cardinals, which as a body elects a new pope. The duties of the cardinals include attending the meetings of the College and making themselves available individually or collectively to the pope if he requests their counsel. Most cardinals have additional duties, such as leading a diocese or archdiocese or running a department of the Roman Curia.

A cardinal's other main function is electing the pope whenever, by death or resignation, the seat becomes vacant. In 1059, the right of electing the pope was reserved to the principal clergy of Rome and the bishops of the seven suburbicarian sees. During the sede vacante, the period between a pope's death and the election of his successor, the day-to-day governance of the Church as a whole is in the hands of the College of Cardinals. The right to enter the conclave of cardinals who elect the pope is now limited to those who are not over 80 years old on the day of the pope's death or resignation.

The term "cardinal" at one time applied to any priest permanently assigned or incardinated to a church, or specifically to the senior priest of an important church, based on the Latin cardo (hinge), meaning "principal" or "chief". The term was applied in this sense as early as the ninth century to the priests of the tituli (parishes) of the diocese of Rome. In the twelfth century the practice of appointing ecclesiastics from outside Rome as cardinals began, with each of them being assigned a church in Rome as his titular church, or being linked with one of the suburbicarian dioceses, while still being incardinated in a diocese other than that of Rome.