Pontifex maximus: the Roman high priest.
The pontifex maximus was not a real magistrate: he did not serve for a fixed period but for life, and he remained, officially, a citizen. As the title suggests, the pontifex maximus was 'the greatest' or chairman of the college of the pontifices, 'priests'. They were responsible for the Roman state cult as a whole and for several cults in particular, viz. the cults that had no priestly college of their own (such as the augures, the decemviri sacris faciundis and the fetiales).
The number of pontifices continued to grow. There were originally five 'real' pontifices, after the Lex Ogulnia (300/299 BCE) nine, after Sulla fifteen, and after Julius Caesar sixteen. Another member was the rex sacrorum ('king of the sacrifices') who performed the religious acts that the king had usually done. There were three (later 15) flamines, special priests for the main gods, and there were three mysterious pontifices minores. Finally, the high priest was also responsible for the eighteen priestesses of the goddess Vesta. This may have been his most important duty, and it comes as no surprise that the residence of the pontifex maximus, the domus publica, was next to the monastery of these women.
The main task of the pontifices was to maintain the pax deorum, the 'peace with the gods'. To obtain this goal, they gave advise to the magistrates, interpreted the omens, controlled the calendar and oversaw funerals. The pontifex maximus was responsible for a large collection of omens (annales maximi); every year, he wrote down the celestial and other signs, and added the events that had followed the omens, so that future generation would be able to better understand the divine will.
Until the Lex Ogulnia, all pontifices were patricians; this law introduced the possibility that plebeians were to be pontifex as well. The pontifex maximus was elected by the comitia tributa, an assembly of the people that was divided into voting districts. After 104 BCE, the ordinary pontifices were also elected - until then, they had been coopted.
Julius Caesar was elected pontifex maximus in 63 BCE and kept the office until his death. The house where he spent the night before he was killed, was the domus publica. After his death, Marcus Aemilius Lepidus became pontifex maximus (44-12 BCE); when he died, the emperor Augustus became responsible for the state cult. He also put an end to the election of the pontifices. From then on, a position in the college of pontifices was a sign of special imperial favor, comparable to a decoration in our age.
The word pontifex is sometimes explained as 'bridge builder', but is in fact related to the Etruscan word pont, 'road', and means something like 'preparer of the road'. The pope still calls himself pontifex maximus.
Because the pontifex maximus was not a real magistrate, he was not allowed to wear the toga with the purple border. However, he could be recognized by the iron knife (secespita).
In 381, the Roman emperor Gratian was the first to decline to use the title of pontifex maximus. It was accepted by the pope.