Sclerosis is a medical word for a disease, or a change, that makes an organ or a soft part of the body go hard. The tissue thickens and stiffens when it should stay soft. You almost never hear sclerosis alone in ordinary talk; it usually sits inside a longer name. Multiple sclerosis (often MS) affects the nerves. Arteriosclerosis is the hardening of the arteries. Doctors use the word; everyday English just names the specific illness. There is a looser newspaper use too — bureaucratic sclerosis — for a system that has gone stiff and slow, but that is not the core medical sense.