1. a firmly held, irrational idea or belief that is maintained despite evidence to the contrary. It may take the form of a delusion and become an obsession. Also called fixed belief; fixed idea.

2. a subconscious unit of mental processing (see automatism) that has become split off or dissociated from consciousness and, as a result, interferes with the normal processing of information. In some theories, this is considered a primary mechanism for the symptoms of hysteria. [proposed by Pierre Janet]