Ever heard of this?
It's basically when everything you think you know, changes. The transformation is usually brought about by specific agents of change (got that term from the article it's linked to; worth reading).
A friend recently told me she didn't think I was an introvert.
That was my agent of change. Me? Not an introvert? I've lived with this image of myself from high school and until my friend said something, I didn't realize how I've changed.
I'm still quiet. I'm still somewhat a loner who gets exhausted after any social event. But I also like to get out and laugh loud. Her comment completely transformed the way I view myself. It surprised me.
In writing, our main characters need to have a paradigm shift. Either at the beginning of the story, in which the character's new view leads to the story taking its path, or at the end, in which the character's shift resolves the story.
Where else do you think paradigm shifts happen in a book? Do you think they're important to the character arc? Have you ever believed something about yourself, only to discover your belief was false or distorted?