catastrophe

noun4

1. •
   Any large and disastrous event of great significance.

   • The Chernobyl disaster was a catastrophe.
2. •
   A disaster beyond expectations.
3. •
   The dramatic event that initiates the resolution of the plot; the dénouement.

   • Pat : he comes like the Catastrophe of the old Comedie : my Cue is villanous Melancholly, with a sighe like Tom o’ Bedlam. — 1603
4. •
   A type of bifurcation, where a system shifts between two stable states.