Dementia

n. mental impairment due to old age or illness (Medical)

Dementia (from Latin de- "apart, away" + mens (genitive mentis) "mind") is the progressive decline in cognitive function due to damage or disease in the brain beyond what might be expected from normal ageing.Particularly affected areas may be memory, attention, language, and problem solving. Especially in the later stages of the condition, affected persons may be disoriented in time (not knowing what day of the week, day of the month, month, or even what year it is), in place (not knowing where they are), and in person (not knowing who they are).
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Dementia is a loss of mental ability severe enough to interfere with normal activities of daily living, lasting more than six [more]Dementia - Community and Resources

Noun
1. mental deterioration of organic or functional origin
(synonym) dementedness
(hypernym) insanity
(hyponym) alcoholic dementia, alcohol amnestic disorder, Korsakoff's psychosis, Korsakoff's syndrome, Korsakov's psychosis, Korsakov's syndrome, polyneuritic psychosis

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