A work of fiction may be considered post-Apocalyptic when its setting or plot features a world in which civilization as we (or the characters) know it has ceased to exist. Many of these stories are set in a near, often dystopian future, or in alternate realities.
Although the term Apocalypse has its origin in Christian religion and refers to a Revelation made by God, over time the definition of the word has expanded, and may now also indicate a more secular end to an age or era such as by plague or war, or the End of Times as foretold in other religions.
Although post-apocalyptic fanfiction (also known as Apocafic) has always proved relatively popular in the fandom, there was a resurgence of interest in the genre following the revelations in late Season Two and Season Three about Sam's possible role as leader in a demonic war against humanity.
In Season Four, post apocalypse was Kripked, in that the apocalypse officially entered the Canon of the Show. In the episode 5.04 The End, Dean is transported into a future where demons have brought on the Apocalypse, partly through the use of the Croatoan virus. The episode spawned a sub-genre of apocalypse fic centered on the events and characters from that episode.