"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 aging. Although dementia is far more common in the geriatric population, it may occur in any stage of adulthood. This age cutoff is defining, as similar sets of symptoms due to organic brain dysfunction are given different names in populations younger than adulthood (see, for instance, developmental disorders).

In dementia, affected areas in cognition may be memory, attention, language, and problem solving. Higher mental functions are affected first in the process. 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).

Symptoms of dementia can be classified as either reversible or irreversible depending upon the etiology of the disease. Less than 10 percent of cases of dementia are due to causes which may presently be reversed with treatment. Of these cases almost 100% are elderly people. Dementia is a term for a non-specific illness syndrome (set of symptoms) which is caused by many different specific disease processes, in the same way that symptoms of organ dysfunction such as shortness of breath, jaundice, or pain are attributable to many etiologies.

Without careful assessment of history, the short-term syndrome of delirium can easily be confused with dementia, because many of the symptoms of these are also present in dementia. Some mental illnesses including depression and psychosis may also produce symptoms which must be differentiated from both delirium and dementia."

Ref: http://www.wikipedia.org