Dementia is a term used to describe a number of physical disorders of the brain arising from different causes. They all have in common a decline in mental function, especially memory.
Dementia interferes with daily activities and social relationships.
Dementia is usually progressive, irreversible and ultimately fatal. Also common is vascular dementia in which part of the brain dies off because of blood supply failure to the brain. There are a number of other much less common types of dementia. For information look at the Alzheimer's Disease International site. In 1906 a German neurologist, Dr. Alois Alzheimer, described changes in the brain tissue of a woman who had died of what was thought to be an unusual mental illness.