Self-deception is a state of preserving a wrong belief, often facilitated by denying or rationalizing away the relevance, significance, or importance of opposing evidence and logical arguments. Beliefs supported by self-deception are often chosen for reasons other than how closely those beliefs approximate truth.
On LessWrong, a common distinction is between beliefs as expectation-controllers and other things people commonly label as beliefs. When these different things conflict, a person is said to have engaged in self-deception.
An example from No, Really, I've Deceived Myself:
When this woman was in high school, she thought she was an atheist. But she decided, at that time, that she should act as if she believed in God. And then—she told me earnestly—over time, she came to really believe in God.
So far as I can tell, she is completely wrong about that. Always throughout our conversation, she said, over and over, "I believe in God", never once, "There is a God." When I asked her why she was religious, she never once talked about the consequences of God existing, only about the consequences of believing in God. Never, "God will help me", always, "my belief in God helps me". When I put to her, "Someone who just wanted the truth and looked at our universe would not even invent God as a hypothesis," she agreed outright.
She hasn't actually deceived herself into believing that God exists or that the Jewish religion is true. Not even close, so far as I can tell.
On the other hand, I think she really does believe she has deceived herself.
Part of How To Actually Change Your Mind sequence