Definition of "Mahdi"

"Of course, my Lord. You asked what they were shouting, though. It was Mahdi! They directed the term at the young master. When they-""At Paul?""Yes, my Lord. They've a legend here, a prophecy, that a leader will come to them, a child of a Bene Gesserit, to lead them to true freedom. It follows the familiar messiah pattern."

1965, Frank Herbert, Dune‎ (Fiction), pages 100–101

Al-Afghani was a polyglot Persian who became an international agitator, aspiring [...] to unify the Muslim masses behind the Caliph (or even the Mahdi) and to become himself the Luther of an Islamic reformation.

2012, Piers Brendon, ‘Beginning the Dissent’, Literary Review, volume 401