THE REASON WHY HAZRAT MAHDI (PBUH) IS CALLED 'AL-MAHDI' AND 'AL-QAIM'

6- Ghaibat Tusi: It is narrated from Fadhl from Musa bin Sadan from Abdullah bin Qasim Hadhrami from Abi Saeed Khurasani that he said:

I asked Imam Ja’far Sadiq (a.s.), “Is the Mahdi and the Qaim the same?” He said: “Yes.”
I asked, “For what reason has he been named the Mahdi?”

He said: “Because he will be guided to every hidden matter. And he is named Qaim, because he will rise after his name dies (meaning after people will lose hope of his appearance and claim that he won't come). He will establish a mighty enterprise.” 

The Promised Mahdi, Allamah Muhammad Baqir al-Majlisi, English Translation of Bihar al-Anwar, Vol. 13 (Old edition)/ Vol. 51-52-53 (New edition); Kitab al-Ghaybah, Imam Mahdi - The Twelfth Imam of the Twelver Shia Part I, Translator, Sayyid Athar Husain S.H.Rizvi), p.54