The Pontifex Maximus ("Chief Bridge-builder") is the title of the highest ranking priest of Rome. It was instituted in 509 BC, at the same time as the Roman Republic, and for the next 500 years was an elected office, most often held by a powerful senator. It was held by Julius Caesar from 60 BC to 44 BC, and was later given to the Emperor Augustus. After Augustus the office was held by the Roman emperors, until it was relinquished by the Emperor Gratian in the fourth century and given to the Bishop of Rome, thus granting him temporal power over Roman religion.
It has been held by every pope since, and is the legal, technical basis of the pope's authority.