Final answer:
The head of an archdiocese is known as an Archbishop, which differs from positions like Cardinal, Bishop, or Papal Legate. The Pope, who is the Bishop of Rome, is recognized as the highest authority in the Church.
Step-by-step explanation:
The head of an important and often older diocese, called an archdiocese, is known as an Archbishop. An Archbishop typically oversees a significant diocesan community which holds a considerable amount of influence and historical importance within the Church's hierarchy.
This role should not be confused with that of Cardinal, Bishop, or Papal Legate, as each of these holds different positions and responsibilities within the Christian ecclesiastical structure. The title of Pope, for instance, is reserved for the Bishop of Rome, who became acknowledged as the spiritual father and highest authority in the Church, a concept stemming from the 'Petrine Succession'.