What is the name of the large church leader of Rome in Western Christianity?

The story behind the answer

As bishop of Rome, the supreme pontiff was recognized in Western Christianity as the universal pastor and head of the Roman Catholic Church, viewed as the successor of Saint Peter—hence his primacy over other bishops. By contrast, the Patriarch of Constantinople led the Eastern Orthodox Church, the Archbishop of Canterbury headed the English church (and, after the Reformation, Anglicanism), and a Cardinal Vicar merely administers the Diocese of Rome on the pontiff’s behalf.

In the Middle Ages, this office wielded immense spiritual and political clout: crowning Charlemagne emperor in 800, arbitrating disputes, initiating Crusades, and ruling the Papal States. Conflicts like the Investiture Controversy and the Avignon Papacy reveal how central this authority was to medieval Europe.