Bishop of Rome
The Bishop of Rome is one of the titles of the Pope, because he is the spiritual leader of the Roman diocese as well as leader of the whole of the Catholic Church. By contrast, the Orthodox churches believe in a number of different bishops, such as the Bishop of Antioch, who are co-equal in their positions of Church leadership; there is no Pope figure in Eastern Orthodox Christianity.
The term has been used frequently throughout the history of the Christian Church. The city of Rome is also thought of as the geographical heart of the Catholic Church.
Pope Leo the Great, in the mid 400s, received an edict from the Roman Emperor acknowledging the leadership of the Pope, the Bishop of Rome, over all other bishops. This has never been accepted by the Eastern Orthodox Church or by the Protestant churches.
The Roman Catholic Church believes that the Bishop of Rome (the Pope) was made the leader of Christendom by Christ through a promise made to Saint Peter and then transferred to those who succeeded him as bishops of Rome. The Emperor's edict was seen as merely putting a stamp of approval upon what already was. The many Christian churches that are independent of the Pope customarily believe that the Bishop of Rome tried to elevate himself above all other bishops and sought imperial approval to do so.
The title, Bishop of Rome, is not as heavily used today as it had been in ancient and medieval times, with the term Pope receiving more usage.