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]

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "dementia":
   aberration, abnormality, alienation, brain damage, brainsickness,
   clouded mind, craziness, daftness, dementedness, derangement,
   disorientation, distraction, folie, furor, insaneness, insanity,
   irrationality, loss of mind, loss of reason, lunacy, madness,
   mania, mental deficiency, mental derangement, mental disease,
   mental disorder, mental disturbance, mental illness,
   mental instability, mental sickness, mind overthrown, mindsickness,
   oddness, pixilation, possession, queerness, rabidness,
   reasonlessness, senselessness, shattered mind, sick mind, sickness,
   strangeness, unbalance, unbalanced mind, unsaneness, unsound mind,
   unsoundness, unsoundness of mind, witlessness