Dementia manifests as a set of related symptoms, which usually surface when the brain is damaged by injury or disease. The symptoms involve progressive impairments to memory, thinking, and behavior, which negatively impacts a person's ability to function and carry out everyday activities. Aside from memory impairment and disruption in thought patterns, the most common symptoms include emotional problems, difficulties with language, and decreased motivation. Dementia is not a disorder of consciousness, and consciousness is not usually affected.

The symptoms may be described as occurring in a continuum over several stages. A diagnosis of dementia requires a change from a person's usual mental functioning and a greater cognitive decline than that due to normal aging. Several diseases and injuries to the brain, such as a stroke, can give rise to dementia. However, the most common cause is Alzheimer's disease, a neurodegenerative disorder.