English Reference
noun

delusion

n.
US /dɪˈɫuʒən/ UK /dɪlˈuːʒən/ delu·sion UK spelling delusion's, delusion's

  1. 01 C1 Advanced Formal

    a belief in something that is not true, which you keep even when people show you proof that you are wrong.

    a firm, fixed belief in something false, maintained despite incontrovertible evidence to the contrary.

    Simple

    He was under the delusion that he could win the race without any training.

    In context

    The patient suffered from the persistent delusion that he was being monitored by a foreign intelligence agency despite all evidence to the contrary.

    Usage

    Often used with the preposition 'under' as in 'under the delusion that'.

    Pitfall
    He has a delusion of his own importance. He has delusions of grandeur.

    When describing someone who thinks they are much more important than they really are, the fixed phrase is 'delusions of grandeur'.

  2. 02 C2 Proficiency FormalLiterary

    the act of tricking someone or the state of being tricked into believing something false.

    the process of deluding or the state of being deluded; the act of deceiving oneself or others.

    Simple

    The company's success was a total delusion created by dishonest accounting.

    In context

    The political campaign was built on a foundation of public delusion, masking the true economic consequences of the proposed policies.

Iran’s deputy foreign minister, Kazem Gharibabadi, dismissed Trump’s comments as “delusions”.

The Guardian· 22 Aug 2026

Whether that’s his delusion or whether he just actually exists in this insane, cartoon world is for the audience to decide.

The Guardian· 20 Aug 2026

From Latin dēlūsiō.

Etymology adapted from Wiktionary, available under CC BY-SA 4.0.

This entry
Level C1 · Advanced
Frequency Frequent
Updated