How is a hallucination defined?

A hallucination is defined as a perception that occurs without any external stimuli. This means that individuals experiencing hallucinations perceive things—such as sounds, sights, or even smells—that are not actually present in the environment. Hallucinations can occur in various sensory modalities, but they do not result from real external stimuli.

This definition contrasts with other options provided. For example, hearing voices during sleep typically refers to vivid dreams rather than hallucinations, which are experienced while one is awake. Seeing images in dreams indicates the experience of dream-related imagery, which does not reflect reality in the waking state. Encoding memories pertains to the cognitive process of transforming experiences into long-term memory storage, which is unrelated to the phenomenon of perceiving things that are not there. Thus, the correct answer encapsulates the essence of hallucinations as perceptions devoid of external origins.