Stephen Hawking had amyotrophic lateral sclerosis, commonly abbreviated as ALS. In the United Kingdom, the closely related term motor neurone disease, or MND, is also commonly used for his condition.
Hawking was diagnosed in 1963, at the age of 21, with an early-onset, slow-progressing form of motor neuron disease. Doctors initially expected him to live for only a few years, but his illness progressed unusually slowly. He continued his scientific work for more than five decades after the diagnosis.
ALS damages motor neurons, the nerve cells that control voluntary muscle movement. As the disease advanced, Hawking gradually lost the ability to move and later to speak. He used a wheelchair and communicated through a speech-generating computer system, eventually controlling it with a muscle in his cheek.
A common mix-up is choosing Parkinson's disease, multiple sclerosis or Huntington's disease because all can affect movement or the nervous system. Those are different conditions. ALS is the standard quiz answer, although British sources often describe Hawking's illness as motor neurone disease.