'Great Tribulation' in the Bible

In the third year of Cyrus king of Persia a message was revealed to Daniel, who was named Belteshazzar; and the message was true and it referred to great conflict (warfare, misery). And he understood the message and had an understanding of the vision.

For at that time there will be a great tribulation (pressure, distress, oppression), such as has not occurred since the beginning of the world until now, nor ever will [again].

I said to him, “My lord, you know [the answer].” And he said to me, “These are the people who come out of the great tribulation (persecution), and they have washed their robes and made them white in the blood of the Lamb [because of His atoning sacrifice].