In Recapture the Rapture, Jamie Wheal lays out and differentiates some of the terms we use to refer to the cataclysmic end of days. Here we'll embellish and expand that a bit.
Eschaton: the final event in the divine plan; the end of the world
Apocalypse: from Greek apocalypsis, the unveiling or revealing, ostensibly of the Big True-true; aka Revelation
Armageddeon: a contraction of Har Megiddo, a mountain outside of Haifa that is to host the ultimate showdown between Good and Evil.
Rapture: big 'R' Rapture is a story of impending cataclysm for the many and joyful redemption of the few; little 'r' rapture means intense bliss or fulfillment
Cataclysm: a large-scale violent event in the natural world, like a big flood; from Greek kataklusmos deluge, from kata- down + kluzein to wash.
Holocaust: originally a Jewish sacrificial offering that was burned completely on an altar; from Greek holokauston, from holos whole + kaustos burnt (from kaiein burn)
Calamity: disaster and distress, from the Latin calamitas
Catastrophe: a sudden downturn, from Greek katastrophΔ overturning, sudden turn, from kata- down + strophΔ turning