1. The loss, usually progressive, of cognitive and intellectual functions, without impairment of perception or consciousness; caused by a variety of disorders, (structural or degenerative) but most commonly associated with structural brain disease.
2. An acquired organic mental disorder with loss of intellectual abilities of sufficient severity to interfere with social or occupational functioning: Dementia is a dysfunction which is multifaceted and involves memory, behavior, personality, judgment, attention, spatial relations, language, abstract thoughts, and other functions.

The intellectual decline of dementia is usually progressive and it initially does not interfere with the level of consciousness.

A form of brain disorder marked by unusually accelerated and progressive, irreversible mental deterioration: Senile dementia is often associated with declining people who appear to experience memory loss, forgetfulness, and disorientation.

Chronic brain disorder with the loss of intellectual functioning resulting from a severe cerebral injury.