In drama, particularly the tragedies of classical antiquity, the catastrophe is the final resolution in a poem or narrative plot, which unravels the intrigue and brings the piece to a close.
In a simple catastrophe, there is no change in the state of the main characters, nor any discovery or unravelling; the plot being only a mere passage out of agitation, to quiet and repose.
To be probable, it must spring out of the subject itself; not affected by means of marks or tokens, rings, bracelets, or by a mere recollection, as is frequently done both in ancient and modern times.
Aristotle, for example, preferred a shocking catastrophe, rather than a happy one; in that regard, the moving of terror and pity, which is the aim of tragedy, is better effected by the former than the latter.
[1] René Le Bossu, a 17th-century French critic, divides the catastrophe, at least with regards to epics, into the unravelling, or denouement, and the finishing, or achievement; the latter of which he makes the result of the former, and to consist in the hero's passage out of a state of trouble and agitation, to rest and quiet.