Dementia isn't a specific disease. Instead, dementia describes a group of symptoms affecting memory, thinking and social abilities severely enough to interfere with daily functioning.
Though dementia generally involves memory loss, memory loss has different causes. So memory loss alone doesn't mean you have dementia.
Alzheimer's disease is the most common cause of a progressive dementia in older adults, but there are a number of causes of dementia. Depending on the cause, some dementia symptoms can be reversed.
Dementia care at Mayo Clinic