Mahdi

It is the most well known title of the Imam among all Muslim sects.2
Shaykh Toosi has narrated in his Ghaibat3 that Abu Saeed Khorasani asked Imam Ja’far Sadiq (a.s.) why the Imam is named as Mahdi. He replied: Because he will guide to every hidden matter. Shaykh Mufeed has said in Irshaad,4 that Imam (a.s.) also said: The Qaaem is named as Mahdi, because he will guide them to matters, which they would have forgotten. Yusuf bin Yahya Salmi says in Iqdud Dorar fee Akhbaaril Imaamil Muntazar5 that Imam Muhammad Baqir (a.s.) said: Mahdi is called as Mahdi, because he will guide to secret matters and he will bring out Taurat and Injeel from the land called as Antioch.