There are different ways to define multiple sclerosis. The official definition, from the National MS Society, goes like this:
Multiple sclerosis (or MS) is a chronic, often disabling disease that attacks the central nervous system (CNS), which is made up of the brain, spinal cord, and optic nerves. Symptoms may be mild, such as numbness in the limbs, or severe, such as paralysis or loss of vision. The progress, severity, and specific symptoms of MS are unpredictable and vary from one person to another.
Sure, yes, very official and sciencey. However, the definition that I use most often goes like this:
Multiple sclerosis is a big pain in the ass.
Short, sweet, covers it in just a few words. MS sucks. I am living with it as best as I can.