Biblical end times is referred to as the Tribulation, 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 of 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). 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.
It is important at this point to emphasize that the Tribulation and the Great Tribulation are not synonymous terms. Within eschatology (the study of future things), the Tribulation refers to the full seven-year period while the “Great Tribulation” refers to the second half of the Tribulation.
It is Christ Himself who used the phrase "Great Tribulation" with reference to the last half of the Tribulation.
In Matthew 24:21, Jesus says, "For then there will be a great tribulation, such as has not occurred since the beginning of the world until now, nor ever shall."
In this verse Jesus is referring to the event of Matthew 24:15, which describes the revealing of the abomination of desolation, the man also known as the Antichrist.
Also, Jesus in Matthew 24:29-30 states,
29 "Immediately after the tribulation of those days shall the sun be darkened, and the moon shall not give her light, and the stars shall fall from heaven, and the powers of the heavens shall be shaken:
30 And then shall appear the sign of the Son of man in heaven: and then shall all the tribes of the earth mourn, and they shall see the Son of man coming in the clouds of heaven with power and great glory."
In this passage, Jesus defines the Great Tribulation (v.21) as beginning with the revealing of the abomination of desolation (v.15) and ending with Christ's second coming (v.30).
Other passages that refer to the Great Tribulation are Daniel 12:1, which says, "And at that time shall Michael stand up, the great prince which standeth for the children of thy people: and there shall be a time of trouble, such as never was since there was a nation even to that same time: and at that time thy people shall be delivered, every one that shall be found written in the book."
Also referring to the Great Tribulation is Jeremiah 30:7, "Alas! for that day is great, so that none is like it: it is even the time of Jacob's trouble, but he shall be saved out of it."
The phrase “Jacob’s trouble” refers to the nation of Israel, which will experience persecution and natural disasters such as have never before been seen.
Considering the information Christ gave in Matthew 24:15-30, the beginning of the Great Tribulation has much to do with the abomination of desolation, an action of the Antichrist. In Daniel 9:26-27, we find that this man will make a "covenant" (a peace pact) with the world for seven years (one “week”).
26 “And after threescore and two weeks shall Messiah be cut off, but not for himself: and the people of the prince that shall come shall destroy the city and the sanctuary; and the end thereof shall be with a flood, and unto the end of the war desolations are determined.
27 And he shall confirm the covenant with many for one week: and in the midst of the week he shall cause the sacrifice and the oblation to cease, and for the overspreading of abominations he shall make it desolate, even until the consummation, and that determined shall be poured upon the desolate”.
Halfway through the seven-year period—"in the middle of the week"—we are told this man will break the covenant he made, stopping sacrifice and grain offering, which specifically refers to his actions in the rebuilt temple of the future.
Revelation 13:1-10 gives even more detail concerning the Beast's actions, and just as important, it also verifies the length of time he will be in power. Revelation 13:5 says he will be in power for 42 months, which is three and one-half years, the length of the Great Tribulation.
Revelation offers us the most information about the Great Tribulation. From Revelation 13 when the Beast is revealed until Christ returns and the battle of Armageddon in Revelation 19, we are given a picture of God's wrath on the earth because of unbelief and rebellion (Revelation 16-18).
It is also a picture of how God disciplines and at the same time protects His people, Israel (Revelation 14:1-5) until He keeps His promise to Israel by establishing an earthly kingdom (Revelation 21)