dementia

Dementia, n.

Etymology: [L., fr. demens. See Dement.]

Insanity; madness; esp. that form which consists in weakness or total loss of thought and reason; mental imbecility; idiocy.

Shades of meaning

Fernald has no entry of its own for dementia; he lists it under insanity.

Where he writes about it:

Dementia is a general weakening of the mental powers: the word is specifically applied to senile insanity, dotage. — from insanity