Defining the Terrifying

What makes a situation terrifying?

The distinctions between horror and terror have been explored, particularly in the realm of gothic literature, but I’d like to suggest a simple definition:

A situation is terrifying when (a) it provokes a fight-or-flight response, and (b) neither fight nor flight is possible.

I like this definition because it covers not only the gothic terror of unknown impending doom, but also the melodramatic strapping of people to logs in sawmills (which would seem a lot more terrifying and a lot less cliché in the first person).  It’s a useful framework for building a scene.  It also strikes me as a decent starting point for developing musical themes.