Learner's definition of DELUSION
1 : a belief that is not true : a false idea
He has delusions about how much money he can make at that job.
He is living/laboring under the delusion that he is incapable of making mistakes.
She is under the delusion that we will finish on time.
2 : a false idea or belief that is caused by mental illness
◊ If you have delusions of grandeur, you believe that you are much more important than you really are.
— delusional/dɪˈluːʒənl̟/adjective,[more delusional; most delusional]