1) A pathological process consisting of hardening or fibrosis of an anatomical structure, often a vessel or a nerve.

2) A pathological condition in which a tissue has become hard and which is produced by overgrowth of fibrous tissue and other changes (as in arteriosclerosis) or by increase in interstitial tissue and other changes (as in multiple sclerosis) -- called also hardening.

3) Any of various diseases characterised by sclerosis -- usually used in combination.