Servus Servorum Dei is a Latin phrase meaning The Servant of the Servants of God. This phrase is one of the titles of the Pope. The Pope uses this phrase to refer to himself in his writings.
History
Pope St. Gregory I was the first pope to use this title to refer to the office of the Pope. Some of his sucessors used the phrase off and on for the several hundred years, but it was not regularly used until the 9th century. Other civil rulers at times used this title as well, but after the 12th century it came to be used exclusively by the Pope.
In the years following the Second Vatican Council, Popes have used the concept of Servus Servorum Dei to help in making their office a simpler and less regal office. Pope Paul VI stopped using the Papal Tiara, and none of his successors have ever worn the tiara. Both John Paul I and John Paul II had dispensed with the Papal Coronation ceremony, instead opting for a simple inauguration ceremony. Instead of receiving the Papal Tiara, both men received the pallium during their installation ceremonies. Also, both men had dispensed with using the royal 'we' in speech, and instead opted for the singular I.