Synonyms for Delusion (same or very similar meaning)
WordNet sense 1 ((psychology) an erroneous belief that is held in the face of evidence to the contrary):
psychotic belief
WordNet sense 2 (a mistaken or unfounded opinion or idea):
hallucination
WordNet sense 3 (the act of deluding; deception by creating illusory ideas):
illusion, head game
From the ODE community, based on WordNet
add/edit