Delusion.
Synonyms:
error, fallacy, hallucination, illusion, phantasm.
A delusion is a mistaken conviction, an illusion a mistaken perception or inference. An illusion may be wholly of the senses; a delusion always involves some mental error. In an optical illusion the observer sees either what does not exist, or what exists otherwise than as he sees it, as when in a mirage distant springs and trees appear close at hand. We speak of the illusions of fancy or of hope, but of the delusions of the insane. A hallucination is a false image or belief which has nothing, outside of the disordered mind, to suggest it; as, the hallucinations of delirium tremens. Compare DECEPTION; INSANITY.
Antonyms:
actuality, certainty, fact, reality, truth, verity.