The English word TRIBULATION (Strong's Concordance #G2347) means distress, misery, affliction and / or persecution. This word is used a number of times in the New Testament to describe a time of testing, trouble and suffering which is prophesied to come upon the Earth before the Second Coming of Christ.
"After these things I (the apostle John) looked, and behold, a great multitude which no one could number, of all nations, tribes, peoples, and tongues, standing before the throne and before the Lamb, clothed with white robes, with palm branches in their hands . . .
"Then one of the elders answered, saying to me, 'Who are these arrayed in white robes, and where did they come from?' And I said to him, 'Sir, you know.' So he said to me, 'These are the ones who come out of the GREAT TRIBULATION, and washed their robes and made them white in the blood of the Lamb.'" (Revelation 7:9, 13-14, NKJV throughout)
According to many prophecy teachers, this is not just referring to an indeterminate time period, but rather to a specific time period they call the Tribulation. The most common speculation, based on comparing prophetic passages in the Bible, is that it will last for seven years. Some of these teachers refer to the whole seven years as the Great Tribulation. Others prefer not to consider the first half of the seven years as being part of this period but only the second half - when circumstances on Earth become even more horrifying and miserable.
Additionally, many try to figure out WHEN this time of global suffering and trials will occur. What is known as Jesus' Olivet prophecies, recorded in Matthew 24, Mark 13 and Luke 21, are used to help determine exactly when the end will come and the Second Coming occurs:
"And Jesus answered and said to them: 'Take heed that no one deceives you. For many will come in My name, saying, ‘I am the Christ,' and will deceive many. And you will hear of wars and rumors of wars. See that you are not troubled; for all these things must come to pass, but the end is not yet. For nation will rise against nation, and kingdom against kingdom. And there will be famines, pestilences, and earthquakes in various places. All these are the beginning of sorrows. Then they will deliver you up to tribulation and kill you, and you will be hated by all nations for My name's sake . . . '
"'Therefore when you see the ‘abomination of desolation,' spoken of by Daniel the prophet, standing in the holy place (whoever reads, let him understand), then let those who are in Judea flee to the mountains . . . And unless those days were shortened, no flesh would be saved; but for the elect's sake those days will be shortened.'" (Matthew 24:4-9, 15-16, 22)