What is dementia? Expand Dementia is a set of symptoms relating to a loss of cognitive ability, with symptoms including memory loss and issues with thinking, problem solving and language. Dementia is caused when the brain is damaged by diseases, which may include Alzheimer's disease. You may hear dementia described as a degenerative condition, meaning that the normal functioning of the brain is gradually being reduced over time. There are many degenerative conditions that affect the brain - each one has its own signs and symptoms and some, but not all, may involve dementia.