DELUSION

[noun]She is suffering from the delusion (= a false belief) that he will marry her. [C + that clause]John is under the delusion that he will be promoted this year. [C + that clause]Unfortunately, he is labouring under the delusion (= believes wrongly) that his students enjoy his lessons. [C + that clause]I think that it's delusion (= deceit) on the part of the government to promise that they won't increase taxes. [U]Diane has always suffered from delusions of grandeur (= has always thought that she is more important than she is).