1. a generalized, pervasive deterioration of memory and at least one other cognitive function, such as language and an executive function, due to a variety of causes. The loss of intellectual abilities is severe enough to interfere with an individual’s daily functioning and social and occupational activity. In DSM–IV–TR, dementias are categorized according to the cause, which may be Alzheimer’s disease, cerebrovascular disease (see vascular dementia), Lewy body dementia, Pick’s disease, Parkinson’s disease, Huntington’s disease, HIV infection (see AIDS dementia complex), Creutzfeldt–Jakob disease, head injury, alcoholism (see alcohol-induced persisting dementia), or substance abuse (see substance-induced persisting dementia). Brain tumor, hypothyroidism, hematoma, or other conditions, which may be treatable, can also cause dementia. The age of onset varies with the cause but is usually late in life. When occurring after the age of 65, it is termed senile dementia, and when appearing before 65, it is called presenile dementia, although these distinctions are becoming obsolete because its manifestations are the same no matter the age of onset. Dementia should not be confused with age-associated memory impairment or mild cognitive impairment, which have a much less deleterious effect on day-to-day functioning. DSM–5 has subsumed dementia and amnestic disorder into the diagnostic category major neurocognitive disorder, although it still accepts the term dementia where commonly used.

2. historically, loss at ability to reason due to mental illness, diseases such as neurosyphylis (see general paresis), or advanced age (senility). [Latin, “out of mind”]