hallucination

hallucination — noun

1. a sensory event during which a person sees, hears, feels, or smells things with

1.nounB2
Definition

a sensory event during which a person sees, hears, feels, or smells things with no basis in external reality, typically triggered by a medical condition, mental illness, a high fever, or drug use

Example

Ritu experienced vivid auditory hallucinations after developing a high fever.

auditory / visual / olfactory hallucination — specifying the sensory type

Layla had visual hallucinations of people not there while awake for two weeks.

Common collocations
  • auditory hallucination
  • visual hallucination
  • tactile hallucination
  • olfactory hallucination
  • experience a hallucination
  • suffer from hallucinations
  • have hallucinations
  • cause hallucinations
  • trigger hallucinations
Synonyms
  • delusion

    A delusion is a fixed false belief (e.g., thinking you are famous), not a sensory experience. Delusions involve thoughts, not senses.

  • mirage

    A mirage is a visual illusion caused by atmospheric conditions (e.g., water on a hot road), not a medical symptom.

  • apparition

    Apparition suggests a ghostly or supernatural vision, whereas hallucination is a medical or neurological term.

Usage Note

Often modified by a sensory adjective (auditory, visual, olfactory, tactile) to specify which sense is affected. The uncountable form (e.g., 'suffers from hallucination') is less common than the countable form ('had hallucinations').

Common Mistakes

He had an illusion of hearing voices.
He had an auditory hallucination of hearing voices.
An illusion is a misperception of something real; a hallucination has no external basis.

2. the specific thing that someone sees, hears, or feels during a hallucination, ev

2.nounB2
Definition

the specific thing that someone sees, hears, or feels during a hallucination, even though it has no real existence outside their own mind

Example

One hallucination Dewi reported was a large spider crawling across her shoulder.

hallucination of + [noun] — describing the perceived object

Eli kept insisting the person in the corner of the room was not a hallucination.

Common collocations
  • the hallucination of + noun
  • report a hallucination
  • describe a hallucination
  • a frightening hallucination
  • a recurring hallucination
Synonyms
  • phantom

    Phantom often refers to a ghost-like or imagined figure; it is less clinical than hallucination.

  • vision

    Vision can be used for religious or supernatural experiences, while hallucination implies a medical or neurological cause.

Grammar Patterns

hallucination of + noun

Usage Note

Use this sense when referring to what the person actually perceived (the content of the hallucination), as opposed to the experience itself. Typically takes an 'of'-phrase (e.g., 'a hallucination of a face').

3. an incorrect statement or fabricated detail that an artificial intelligence syst

3.nounC1
Definition

an incorrect statement or fabricated detail that an artificial intelligence system creates and presents as if it were reliable, often sounding convincing despite being untrue

Example

The chatbot produced a hallucination when it claimed a famous scientist was still alive.

AI system + produce + hallucination — verb-noun collocation in computing

Rachel deleted the article after discovering it contained several serious hallucinations.

Common collocations
  • AI hallucination
  • produce a hallucination
  • generate a hallucination
  • contain a hallucination
  • a serious hallucination
  • a factual hallucination
Synonyms
  • fabrication

    Fabrication is more general and does not imply AI; a person or a system can fabricate information.

  • confabulation

    In AI research, confabulation is sometimes used synonymously with hallucination, but hallucination is far more common in non-specialist writing.

Antonyms
  • fact

    A fact is verified and true; the opposite of a hallucinated piece of information.

Grammar Patterns

produce / generate + hallucination

Usage Note

In computing contexts, hallucination refers specifically to incorrect AI-generated output; do not use the medical meaning (senses 1-2) when discussing technology. The countable form ('three hallucinations') is common when referring to individual errors.

Common Mistakes

The AI model experienced hallucinations like a human patient.
The AI model generated hallucinations in its output.
In computing, AI systems 'generate' or 'produce' hallucinations; they do not 'experience' them.

4. the general tendency or phenomenon of artificial intelligence systems producing

4.nounC1
Definition

the general tendency or phenomenon of artificial intelligence systems producing false or misleading information, considered as a technical problem that researchers try to solve

Example

AI hallucination remains one of the biggest challenges for developers of language models.

AI hallucination + remains / is — subject pattern in academic contexts

Tunde's research paper explores why hallucination occurs in machine translation systems.

hallucination + occurs in / happens in — describing where the phenomenon appears

Common collocations
  • reduce hallucination
  • prevent hallucination
  • minimize hallucination
  • hallucination in AI
  • the problem of hallucination
  • address hallucination
Synonyms
  • AI confabulation

    A more technical synonym used in academic papers; less common in general technology journalism.

  • model error

    A broader term that covers any type of mistake an AI makes, not just fabricated content.

Antonyms
  • factual accuracy

    Factual accuracy is the opposite property — the quality of being correct and true.

Usage Note

This uncountable sense describes the phenomenon as a whole ('reduce hallucination'), rather than individual errors. It is used in technical discussions about AI reliability, safety, and model improvement. Distinguish from sense 3, which refers to a specific false output.

5. an idea about a situation, a person, or a possibility that has no basis in fact

5.nounC1
Definition

an idea about a situation, a person, or a possibility that has no basis in fact and is held despite a clear lack of supporting evidence, often because the person wants or fears it to be true

Example

The CEO's belief that the firm could not fail was a dangerous hallucination.

hallucination that + clause — specifying the false belief content

Ada dismissed the idea that money alone buys happiness as a comfortable but false hallucination.

Common collocations
  • a dangerous hallucination
  • a comfortable hallucination
  • live under a hallucination
  • a hallucination that...
  • dismiss something as a hallucination
Synonyms
  • delusion

    Delusion is the more common word for a fixed false belief. Hallucination (sense 5) is more poetic or dramatic.

  • fantasy

    Fantasy is usually a pleasant imagined scenario that the person knows is not real; a hallucination in this sense is believed to be true.

  • misconception

    Misconception is a milder, more neutral term for an incorrect understanding.

Antonyms
  • reality

    Reality represents the true state of affairs, the opposite of a false belief.

  • truth

    Truth is what is factually correct, as opposed to an unfounded idea.

Grammar Patterns

hallucination that + clause

Usage Note

This figurative sense is literary or formal. Unlike sense 1, it involves no sensory perception — only a false belief or expectation. For a false belief that is more rigid and resistant to evidence, use 'delusion' instead.

Common Mistakes

He had a hallucination about winning the lottery.
He had a delusion about winning the lottery.
For false beliefs without sensory perception, 'delusion' is more natural in most contexts.

Word family

  • hallucinate
    verbB2
  • hallucinatory
    adjectiveC1
  • hallucinogenic
    adjectiveC1
  • hallucinogen
    nounC1

Synonym discussion

Sense: noun/1

Hallucination differs from delusion in that hallucination involves sensory experience — seeing, hearing, or feeling something that does not exist — while delusion involves a fixed false belief, such as thinking one is being watched or followed. Illusion refers to a misinterpretation of something that is actually there, like a stick appearing bent in water, rather than perceiving something with no external cause at all. Mirage is a specific type of visual illusion caused by atmospheric conditions, typically seen in deserts or on hot roads as what looks like water in the distance. In everyday language, hallucination is the strongest term, implying a genuine break from reality, whereas illusion can be a harmless trick of the eye. For example: 'The patient had a visual hallucination of insects on her skin.' vs 'The mirror created an optical illusion of depth.' vs 'He suffers from the paranoid delusion that everyone is conspiring against him.'

hallucination ↗delusionillusion ↗mirage

Common collocations

Nouns

Adverbs

Phrases

  • suffer from hallucinations
  • experience a hallucination
  • have a hallucination
  • reduce hallucination
  • the problem of hallucination

Prepositions

Etymology

LatinFrenchEnglish

The word hallucination comes from Latin hallucinari, meaning 'to wander in the mind' or 'to dream'. It entered English through French in the 16th century and originally referred to mental wandering or daydreaming. The modern medical meaning — perceiving something that is not there — became standard in the 19th century with the rise of psychiatry and neurology. In the 2020s, the word gained a new technical meaning in computing, where it describes false information produced by artificial intelligence.