Hallucination refers to an abnormal sensory experience that arises in the absence of a direct external stimulus, and which has the qualities of a normal percept and is experienced as real and usually in external space.
Hallucinations may occur in any sensory modality.
Other /More definition:
Hallucination is defined as a report of an experience in the absence of any apparent stimulation. It is an imaginary sensation, such as seeing, hearing, or smelling something that does not exist in the external world.