In the Bible, The Apocalypse - or the End of Days - refers to a time when the Judgement of God will come, and we will all either be brought to Heaven or thrown in Hell for eternity. It's an event of monumental impact on the whole of humanity. However, when people tend to think of The Apocalypse (and the fiction derived from the topic), this is not entirely what we think of.

The rise in popularity of the sub-genre began shortly after WWII with the public awareness of the immediate threat of nuclear war with the USSR and its allies. This conscious fear led to an abundance of fiction that described the world after such a nuclear holocaust. Asimov, Bradbury, Heinlein, Wells, and

To me, the definition has evolved to this: fiction that describes the end of the world as we know it and/or the aftermath thereof.