Wikipedia, citing Leon Festinger, Ph.D., describes cognitive dissonance as follows:
In psychology, cognitive dissonance is the mental stress or discomfort experienced by an individual who holds two or more contradictory beliefs, ideas, or values at the same time, performs an action that is contradictory to one or more beliefs, ideas or values, or is confronted by new information that conflicts with existing beliefs, ideas, or values.
I’m not a mental health professional so I’m free to fine-tune or even elaborate on the definition offered by an extremely well-regarded psychologist. I see cognitive dissonance as evident in situations where people reject facts that contradict long-cherished beliefs rather than reject the beliefs which the facts don’t support.
Cognitive dissonance is obvious in the increasing anger and resentment Clinton supporters are expressing towards Bernie Sanders. Where facts prove beyond peradventure that Hillary Clinton is fundamentally dishonest, a corporate shill, a neo-con, and a race-card player, her supporters project these or comparable negative traits on her truly decent and compassionate opponent. Why have people whose support for Clinton almost certainly stemmed from the best of motives turned so harshly against the better candidate?
The answer is cognitive dissonance. Many of Clinton’s backers feel stress and anxiety when confronted with the truth about her. Rather than blame her for fooling them or themselves for being suckers, they lash out at the one demonstrating the empress has no clothes.