A hallucination is a sensory perception experienced in the absence of an external stimulus, as distinct from an illusion, which is a misperception of an external stimulus.
Hallucinations may occur in any sensory modality - visual, auditory, olfactory, gustatory, tactile, or proprioceptive (sense of balance and position in space).
Psychological research has presented the idea that hallucinations may result from biases in what are known as metacognitive abilities.
These are abilities that allow us to monitor or draw inferences from our own internal psychological states (such as intentions, memories, beliefs and thoughts).
The ability to discriminate between self-generated and external sources of information is considered to be an important metacognitive skill and one which may break down to cause hallucinatory experiences.