An Apocalypse is when someone finds out about something that has been hidden from most people. It comes from the Greek word, Apokálypsis, which means lifting of the veil, or finding out something secret. Often this secret is discovered in a dream or a vision.
Today the word is used to describe Armageddon or the end of the world. This might have come from the words, apokalupsis eschaton, meaning all the secrets will be discovered "at the end of the æon, or age". In Christianity The Apocalypse of John is the Book of Revelation, the last book of the Bible. It describes the end of the world, and the Second Coming of Jesus Christ.