'Great Tribulation' in the Bible

for there shall be then great tribulation, such as was not from the beginning of the world till now, no, nor may be.

'And there came a dearth upon all the land of Egypt and Canaan, and great tribulation, and our fathers were not finding sustenance,

lo, I will cast her into a couch, and those committing adultery with her into great tribulation -- if they may not repent of their works,

and I have said to him, 'Sir, thou hast known;' and he said to me, 'These are those who are coming out of the great tribulation, and they did wash their robes, and they made their robes white in the blood of the Lamb;