Phantom limb pain, also known as limb hallucination pain, refers to when a patient feels that a severed limb is still present and experiences pain in that area.
A person with an amputated hand may still feel the hand being cut, burned, or electrocuted.
Some believe that the essence of "phantom limb pain" is the nerve deficit following the disappearance of a limb. The brain does not receive neural signals from that area and cannot comprehend the fact that the "arm has been amputated," leading it to assume the limb is damaged, thus creating a "warning" signal within the brain.
Of course, there is also a theory that this arises from a compensatory mechanism in the cerebral cortex—after losing part of a limb, the brain mistakenly perceives sensations from other parts as those from the original limb, thus creating significant contradictions between "touch" and other perceptions.
