Dementia is the progressive or chronic decline in cognitive function due to damage or disease in the body beyond what might be expected from normal ageing. Its name is derived from the Latin words 'de' meaning 'apart or away', and 'mens' meaning 'mind'. Although dementia is far more common in older adults, it may occur in any stage of adulthood.

Dementia is a non-specific illness syndrome which may affect 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 not know what day of the week, month, or even what year it is. They may not know where they are, and may also fail to recognise the familiar people around them. Dementia, though treatable, is incurable and no two people will have the same challenges or needs.

Symptoms of dementia can be classified as either reversible or irreversible, depending upon the cause of the disease. Less than 10 percent of cases of dementia are due to causes which may presently be reversed with treatment. Causes include many different specific disease processes. Without careful assessment of history, the short-term syndrome of delirium can easily be confused with dementia, because they have many symptoms in common. Some mental illnesses, including depression and psychosis, may also produce symptoms which must be differentiated from both delirium and dementia.

Demenitia currently affects over 700,000 people in the UK, and this is estimated to rise to 870,000 by the year 2010. Currently 15,000 people under the age of 65 have dementia.