Final answer:
Dementia is a decline in mental ability that affects memory, thinking, and social abilities. The most common cause is Alzheimer's disease, but there are other types of dementia as well.
Step-by-step explanation:
Dementia is a general term used to describe a decline in mental ability severe enough to interfere with daily life. It is not a specific disease but rather a group of symptoms that affects memory, thinking, and social abilities. The most common cause of dementia is Alzheimer's disease, accounting for about 60-80% of cases. Other causes include vascular dementia, Lewy body dementia, frontotemporal dementia, and mixed dementia.