Definition of delusion 

1a: something that is falsely or delusively believed or propagated under the delusion that they will finish on schedule delusions of grandeur

bpsychology: a persistent false psychotic belief regarding the self or persons or objects outside the self that is maintained despite indisputable evidence to the contrary the delusion that someone was out to hurt him also: the abnormal state marked by such beliefs

2: the act of tricking or deceiving someone : the state of being deluded … accused the Bohemian of having practiced the most abominable arts o