Dementia is a syndrome characterized by a progressive decline in cognitive function, including memory, thinking, language, problem-solving, and perception, severe enough to interfere with daily life and activities. It is not a specific disease but rather a group of symptoms caused by various underlying brain disorders, most commonly Alzheimer's disease. Other causes include vascular dementia, Lewy body dementia, and frontotemporal dementia. Symptoms can vary depending on the affected brain regions.