What exactly is the tribulation?
The word "tribulation," used as a prophetic term, comes from Jesus" Olivet discourse, during which He laid out the future course of human history all the way to its climax at His Second Coming. The Lord said, in response to His disciples' question, "What shall be the sign of thy coming, and of the end of the world?"
"For then shall be great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be" (Matthew 24:21).
"Tribulation" is the term used for the general era of God's judgment on a rebellious world of earth-dwellers. It is a time that will begin with Antichrist signing a seven-year covenant guaranteeing Israel's peace and security (Daniel 9:27). This era is kicked off, symbolically, beginning with the first horseman of the Apocalypse (Revelation 6:1).
The "Great Tribulation" is considered to be the last three and one-half years of that seven-year period known as "Daniel's seventieth week." The last three and one-half years is called "the time of Jacob's trouble" in Jeremiah 30:7, when God's most horrific, but totally righteous, wrath is poured directly upon the rebels of planet Earth.
These judgments are portrayed and precisely outlined in Revelation, chapters 6 through 19. The Tribulation will end when Christ returns from heaven to defeat God's enemies at the final battle of Armageddon and set up His millennial kingdom on earth (Revelation 19:11-16).