"In Christian eschatology, the Great Tribulation (Ancient Greek: θλῖψις μεγάλη, romanized: thlîpsis megálē) is a period mentioned by Jesus in the Olivet Discourse as a sign that would occur in the time of the end.[1]"
https://en.wikipedia.org/wiki/Great_Tribulation
"The Tribulation is a future time period when the Lord will accomplish at least two aspects of His plan: 1) He will complete His discipline of the nation Israel (Daniel 9:24), and 2) He will judge the unbelieving, godless inhabitants of the earth (Revelation 6 - 18). The length of the Tribulation is seven years. This is determined by an understanding of the seventy weeks of Daniel (Daniel 9:24-27; also see the article on the Tribulation). The Great Tribulation is the last half of the Tribulation period, three and one-half years in length. It is distinguished from the Tribulation period because the Beast, or Antichrist, will be revealed, and the wrath of God will greatly intensify during this time. Thus, it is important at this point to emphasize that the Tribulation and the Great Tribulation are not synonymous terms."
https://www.gotquestions.org/Great-Tribulation.html
"The Great Tribulation
Speaking of this time before His return to earth, Jesus said: “For then there will be great tribulation, such as has not been since the beginning of the world until this time, no, nor ever shall be. And unless those days were shortened, no flesh would be saved; but for the elect’s sake those days will be shortened” (verses 21-22).
This time of suffering and death is called the Great Tribulation because its severity will surpass all previous periods of suffering experienced throughout human history (verse 21)."
https://lifehopeandtruth.com/prophecy/revelation/great-tribulation/
"The great tribulation will bring the greatest time of trouble ever to come upon mankind. According to Bible prophecy, it will occur during “the last days,” or “the end time.” (2 Timothy 3:1; Daniel 12:4, New American Bible) It will be “a tribulation such as has not occurred from the beginning of the creation that God created until that time, and will not occur again.”—Mark 13:19; Daniel 12:1; Matthew 24:21, 22."
https://www.jw.org/en/Bible-teachings/questions/great-tribulation