According to the Shia and Sunni versions of the Islamic eschatology the Mahdi (مهدي Mahdī, also Mehdi; "Guided One") is the prophesied redeemer of Islam who will stay on earth seven, nine, or nineteen years (depending on the interpretation before the coming of the day, Yaum al-Qiyamah (literally "Day of the Resurrection" or "Day of the Standing"). Muslims believe the Mahdi will rid the world of error, injustice and tyranny alongside Jesus. The concept of Mahdi is not explicitly mentioned in the Qu'ran nor in the Sunni hadiths such as Sahih al-Bukhari which only mention the second coming of Jesus. Many orthodox Sunnī theologians accordingly question Mahdist beliefs but such beliefs form a necessary part of Shīʿī doctrine.
According to scholars Julie Vryhof and Mitchell Uitvlugt, the advent of Mahdi is not a universally accepted concept in Islam and among those that accept the Mahdi there are basic differences among different sects of Muslims about the timing and nature of his advent and guidance. The idea of the Mahdi has been described as important to Sufi Muslims, and a "powerful and central religious idea" for Shia Muslims who believe the Mahdi is the Twelfth Imam, Muhammad al-Mahdi who will return from occultation. However, among Sunni, it "never became a formal doctrine" and is neither endorsed, nor condemned "by the consensus of Sunni Ulama." It has "gained a strong hold on the imagination of many ordinary" self-described orthodox Sunni though, thanks to Sufi preaching. Another source distinguishes between Sunni and Shia beliefs on the Mahdi saying the Sunni believe the Mahdi will be a descendant of the Prophet named Muhammad who will revive the faith, but not necessarily be connected with the end of the world, Jesus or perfection.
The "hdi" of "Mahdi" refers to the Arabic root "هدی" which means "to guide". "Mahdi" is also an Arabic