1. The hardening and thickening of body tissue occurring in patches in the brain or spinal cord, or both: Multiple sclerosis can cause symptoms ranging from numbness and tingling, to paralysis and incontinence. It is also characterized by tremors, failure of coordination, and various nervous and mental symptoms.
2. A chronic illness caused by lesions in the white matter of the central nervous system that degenerate the myelin sheath (insulating layer surrounding vertebrate peripheral neurons), marked by a lack of muscle coordination, muscle weakness, speech problems, and abnormal neurological sensations which include: numbness, tingling, burning, prickling, and hyperesthesia (increased sensitivity), and visual impairments.

The cause of multiple sclerosis is unknown, but it is believed to be an autoimmune disorder, in which the body's immune system mistakenly attacks its own tissues as if they were foreign or not supposed to be there.

The symptoms of multiple scleroses vary, depending on which nerves of the brain and spinal cord lose their protective covers.