What does "sclerosis" mean?

Sclerosis: (Bot.) Hardening of the cell wall by lignification. Cerebro-spinal sclerosis (Med.), an affection in which patches of hardening, produced by increase of the neuroglia and atrophy of the true nerve tissue, are found scattered throughout the brain and spinal cord. It is associated with complete or partial paralysis, a peculiar jerking tremor of the muscles, headache, and vertigo, and is usually fatal. Called also multiple, disseminated, or insular, sclerosis.