Wikipedia
Mujaddid
A mujaddid , is an Islamic term for one who brings "renewal" ( tajdid) to the religion. According to the popular Muslim tradition, it refers to a person who appears at the turn of every century of the Islamic calendar to revive Islam, cleansing it of extraneous elements and restoring it to its pristine purity.
The concept is based not on the Quran but on a famous hadith (Prophetic tradition) recorded by Abu Dawood: Abu Hurairah narrated that the Islamic prophet Muhammad said:
Mujaddid tend to come from the most prominent Islamic scholars of the time, although they are sometimes pious rulers.