delusion (n.)

"act of misleading someone, deception, deceit," early 15c., delusioun, from Latin delusionem (nominative delusio) "a deceiving," noun of action from past-participle stem of deludere (see delude). As a form of mental derangement, "false impression or belief of a fixed nature," 1550s.

Technically, delusion is a belief that, though false, has been surrendered to and accepted by the whole mind as a truth; illusion is an impression that, though false, is entertained provisionally on the recommendation of the senses or the imagination, but awaits full acceptance and may not influence action. Delusions of grandeur, the exact phrase, is recorded from 1840, though the two words were in close association for some time before that.