Source : Webster's Revised Unabridged Dictionary (1913)

Dementia \De*men"ti*a\, n. [L., fr. demens. See {Dement}.]
   Insanity; madness; esp. that form which consists in weakness
   or total loss of thought and reason; mental imbecility;
   idiocy.

Source : WordNet®

dementia
     n : mental deterioration of organic or functional origin [syn: {dementedness}]