Plain English Meaning
A long-term illness that damages the nerves and can cause weakness, vision trouble, balance problems, and tiredness. The FAA does not allow pilots with this condition to fly on a standard medical certificate; they must apply for a special exception.
Full Definition
A chronic, progressive disease of the central nervous system in which the protective covering around nerve fibers (myelin) is damaged, disrupting communication between the brain and the rest of the body. Symptoms can include vision problems, muscle weakness, numbness, balance and coordination difficulties, fatigue, and cognitive changes. Multiple sclerosis is a medically disqualifying condition under FAA standards and requires a special issuance medical certificate to fly.