The Great Tribulation (Greek: θλιψις μεγαλη, thlipsis megalē) is the future period of time in earth's history when suffering and war will become commonplace. It was predicted to happen by God's prophets and His apostles and by Jesus himself (Matthew 24:1-51 , Mark 13:1-37 , John 16:1-33 ), and the Book of Revelation (Revelation 2:22 , Revelation 7:14 ) goes into accurate details concerning the events that will happen during the Great Tribulation.
The Great Tribulation will shortly precede the second coming of Jesus Christ and it will happen when the Antichrist rules the world. The Great Tribulation is mainly believed to last 7 years, but there are different opinions on how long it will last.
Some Christians believe that the Great tribulation is not a specific future event, but rather that it has already happened in the past or is currently being fulfilled right now.