Illusion
Part of speech: noun
Definition: Anything that seems to be something that it is not. A misapprehension; a belief in something that is in fact not true. A magician's trick. The fact of being an illusion (in any of the above senses).
Example sentence: You are priests, not social or political leaders. Let us not be under the illusion that we are serving the Gospel through an exaggerated interest in the wide field of temporal problems.
Delusion
Part of speech: noun
Definition: A false belief that is resistant to confrontation with actual facts.The state of being deluded or misled.That which is falsely or delusively believed or propagated; false belief; error in belief.
Example sentence: No man is happy without a delusion of some kind. Delusions are as necessary to our happiness as realities.