What is the definition or description of: dementia?

Brain Disorder: Dementia is a brain disorder that seriously affects a person’s ability to carry out daily activities. The most common form of dementia among older people is Alzheimer’s disease(AD), which initially involves the parts of the brain that control thought, memory, and language. Vascular Dementia, Lewy Body Dementia are some of the other dementias.

Dementia is a syndrome of brain dysfunction-impairments in memory, executive functioning, visual spatial processing, speech-understanding or expressive, and other areas. there are also functional deficits. Alzheimer's Disease is most common cause though there are others.