The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dementia
    n 1: mental deterioration of organic or functional origin [syn:
         dementia, dementedness]