Patient wants to scratch for itching in his amputated limb is an example of
The phantom limb is the most common organic somatic hallucination of psychiatric origin. In this case the patient feels that they have a limb from which in fact they are not receiving any sensations either because it has been amputated or because the sensory pathways from it have been destroyed. Reference: Fish's Clinical Psychopathology, page 29.
Illusion
Pseudohallucination
Phantom limb hallucination
Autoscopic hallucination
Symptoms and signs in psychiatry and classification