Amyotrophic lateral sclerosis (ALS) is a form of motor neuron disease, a progressive neurodegenerative disease. As the disease develops, motor neurons die and the brain loses its ability to initiate and control muscle movements.
Amyotrophy is the progressive wasting of muscle tissue. Sclerosis is the stiffening of tissue, usually due to replacement with connective tissue.
Stephen Hawking has ALS.