Literally, ‘Raj’at’ means“return” and“reappearance” . In religious terminology, it means to return to life in this world after death. One experiencing ‘Raj’at’, will become alive and would continue to live once again in this material world until death overtakes him.
The holy Qur’an is the main documentary proof on ‘Raj’at’. There, the Almighty Allah has talked about ‘Raj’at’ of prophet Uzair, the youth in the Cave, and a group from Bani-Israel.
About Uzair, we find this matter in Chapter Baqarah: 259, about the Youth in the Cave in Chapter Kahf: 25, about the Bani-Israel group in Chapter Baqarah: 56, about the dead becoming alive by hands of Jesus (a.s.) in Chapter Maeda: 110 and about a group from Shaam in Chapter Baqarah: 243. Here, we shall set forth only about the group of people from Shaam (Syria).
The Almighty Allah says:
“Have you not considered those who went forth from their homes, for fear of death, and they were thousands, then Allah said to them, Die; again He gave them life.”
They were residents of a city in Shaam (Syria)….for fear from plague disease; they fled to a deserted city (all its residents had died of plague). They reached that city and died there instantly. Some time passed and their bodies turned into bones.“Ezekiel” , a Bani-Israel prophet passed besides them. By witnessing this city, he wept and shed tears and prayed: ‘O God, make them alive at this very moment just as Thou have made them die; so that they develop Thy cities and they bear slaves who would worship Thee.’
Here, Allah commanded him to recite Allah’s Exalted and Great Name. He called Allah by His Exalted Name. All became alive and while staring at each other, they praised Allah and said ‘Allah-o-Akbar’ and ‘La-Ilaha-Illallah’.
At that moment,“Ezekiel” said:
اشهد ان الله علي كل شيء قدير (Qur’an, 2:244)
(Tafsir Al-Safa 1: 115 below the verse 244 of Chapter Baqarah.
In various verses, the holy Qur’an talks about ‘Raj’at’ during Hazrat Mahdi’s rule:
“And on the day when We will gather from every nation a party from among those who rejected Our communications, then they shall be formed into groups.” (Qur’an, 27:83)
In interpretation of this verse, we shall narrate a tradition from Tafsir-Kanz-ul-Daqa’eq:
Hammad says: Imam Sadiq (a.s.) said, ‘What do people say about this verse“We will gather from every nation a party” ? I said: ‘they believe it is about resurrection.’ Imam (a.s.) replied: ‘It is not so! Instead, this verse is about ‘Raj’at’. How can it be that Allah will gather on the day of resurrection only a party or group from every nation and leave behind the rest of the people!? (It cannot be so). The verse that talks about resurrection is the one that says: ‘We will gather them and leave not any one of them behind.’
Numerous traditions have come down about ‘Raj’at:
Imam Sadiq (a.s.) says:
“Following the split in the earth, the first one to return to life (Raj’at) in this world would be Hazrat Husain-ibn-Ali (a.s.). Raj’at does not concern every people. Rather, pure believers and absolute polytheists would return to life.” 
To sum up, promise and glad-tiding about Raj’at (return to life) in future and at the time of Hazrat Mahdi’s rule is something having a past record among the past nations and tribes.