Dementia (taken from Latin originally meaning "madness", from de- "without" + ment, the root of mens "mind") is a serious loss of global cognitive ability in a previously unimpaired person, beyond what might be expected from normal aging. It may be static, the result of a unique global brain injury, or progressive, resulting in long-term decline due to damage or disease in the body. Although dementia is far more common in the geriatric population, it can occur before the age of 65, in which case it is termed "early onset dementia".

Dementia is not a single disease, but rather a non-specific illness syndrome in which affected areas of cognition may be memory, attention, language and problem solving. It is normally required to be present for at least 6 months to be diagnosed; cognitive dysfunction that has been seen only over shorter times, in particular less than weeks must be termed delirium. In all types of general cognitive dysfunction, higher mental functions are affected first in the process.