The head of the Roman Catholic Church, elected by cardinals of that church. Normally celibate, although some notable exceptions occur to this during the Middle Ages and Renaissance.
During the middle Ages, the pope could have enormous power (see crusade). Theoretically, there is only one pope. However, several times during the middle Ages there were multiple popes, each of which claimed they were the legitimate and rightful pope. The ones who eventually lost are known as antipopes.
The modern pope lives in Vatican, a religious city state in Italy, but originally the pope lived in Rome, although a few popes and antipopes set up their capital elsewhere (eg Clement V in Avignon).
A pope is properly addressed as "Your Holiness".
A list of mid to late medieval popes can be found at :
Unfortunately this leaves out the antipopes, which can be seen in the briefer listing at: []
Wikipedia has a full list of popes, which also links to a list of antipopes, including one or two nutbars with extra almonds.
It could be observed that popes who are called Pius or Innocent are neither Pious nor Innocent.