What does delusional mean?
Delusional is an adjective describing beliefs that persist despite clear evidence they are false. Its most precise sense belongs to psychiatry, where it characterizes delusions — rigid, unshakable convictions such as believing one is being persecuted or possesses extraordinary powers — as seen in conditions like delusional disorder. In ordinary speech the word has loosened considerably: calling a plan or expectation delusional simply means it rests on wishful, unrealistic thinking rather than on reality, as in 'it would be delusional to expect overnight success.' Derived from the Latin deludere, meaning to mock or play false, the word entered English through the noun 'delusion.' It carries strong negative connotations, implying self-deception or detachment from reality, and should be used carefully: while vivid in casual criticism, its clinical weight means applying it casually to people with genuine mental illness can seem insensitive. Synonyms include deluded, misguided, and unrealistic.
Characterized by or suffering from delusions; maintaining false beliefs despite overwhelming evidence against them.
"The patient's delusional conviction that he was being followed alarmed his family."
This sense is standard in clinical and psychological contexts.
Having ideas or expectations that are wildly unrealistic or based on wishful thinking.
"You'd be delusional to believe the lottery is a retirement plan."
Informal and often hyperbolic in this figurative sense.
The Latin root of 'delusional' literally means 'to play a trick on' — so calling someone delusional is, etymologically, saying they've fallen for their own con.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of delusional
Delusional derives from delusion, which English borrowed in the late Middle Ages from the Latin delusio, meaning 'a mocking or deceiving,' itself formed from deludere ('to play false, to mock') — a combination of de- ('away, off') and ludere ('to play'). Ludere also gives us ludicrous, elude, and illusion (via related forms), making these all distant relatives sharing the same playful root. The adjective delusional emerged in English usage alongside the growth of modern psychology, becoming especially prominent in nineteenth- and twentieth-century psychiatric terminology before spreading into general figurative use.
Related word forms
How delusional is actually used
In psychiatric and medical writing, 'delusional' carries a precise technical meaning tied to clinically defined delusions. In everyday informal use it has softened into hyperbole for anyone with an implausibly optimistic belief, which some consider dismissive of mental illness. The adverbial form 'delusionally' exists but is rare.
Easily confused with delusional
'Delusional' describes persistent false beliefs held against evidence, while 'illusional' relates to illusions—misperceptions of real external stimuli.