Servus Servorum Dei is a Latin phrase meaning Servant of the Servants of God. The phrase is one of the titles of the Pope and is used to refer to the Pope in the beginning address of Papal bulls.
Pope St. Gregory I (pope from 590 to 604) was the first pope to use this title to refer to himself as Pope. The adoption of the title stemmed from a dispute with the Archbishop of Constantinople John the Faster who adopted the title "Ecumenical Patriarch": the humble title "Servant of the Servants of God" countervailed the other's claim of power and eminence against the Bishop of Rome (the Pope). Some of Pope Gregory's successors used the phrase off and on for some centuries, but they did so regularly only from the 9th century. At times, some civil rulers also used this title, but after the 12th century it came to be used exclusively by the Pope.