Dementia is a disorder which manifests as a set of related symptoms, which usually surfaces when the brain is damaged by injury or disease. The symptoms involve progressive impairments in memory, thinking, and behavior, which negatively affects a person's ability to function and carry out everyday activities. Aside from memory impairment and a disruption in thought patterns, the most common symptoms include emotional problems, difficulties with language, and decreased motivation. The symptoms may be described as occurring in a continuum over several stages. Consciousness is not affected. Dementia ultimately has a significant effect on the individual, caregivers, and on social relationships in general. A diagnosis of dementia requires the observation of a change from a person's usual mental functioning and a greater cognitive decline than what is caused by 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. The Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5), has re-described dementia as either a mild or major neurocognitive disorder with varying degrees of severity and many causative subtypes. The International Classification of Diseases (ICD-11) also classes dementia as a neurocognitive disorder (NCD) with many forms or subclasses. Dementia is listed as an acquired brain syndrome, marked by a decline in cognitive function, and is contrasted with neurodevelopmental disorders. Dementia is also described as a spectrum of disorders with causative subtypes of dementia based on a known disorder, such as Parkinson's disease, for Parkinson's disease dementia; Huntington's disease, for Huntington's disease dementia; vascular disease, for vascular dementia; HIV infection, causing HIV dementia; frontotemporal lobar degeneration for frontotemporal dementia; or Lewy body disease for dementia with Lewy bodies, and prion diseases. Subtypes of neurodegenerative dementias may also be based on the underlying pathology of misfolded proteins such as synucleinopathies, and tauopathies. More than one type of dementia existing together is known as mixed dementia.

Many neurocognitive disorders may be caused by another medical condition or disorder that includes brain tumours, and subdural hematoma; endocrine disorders such as hypothyroidism, and hypoglycemia; nutritional deficiencies including thiamine, and niacin; infections, immune disorders, liver or kidney failure, metabolic disorders such as Kufs disease, and some leukodystrophies, and neurological disorders such as epilepsy, and multiple sclerosis. Some of the neurocognitive deficits may sometimes show improvement with treatment of the medical condition.

Diagnosis is usually based on history of the illness and cognitive testing with imaging. Blood tests may be taken to rule out other possible causes that may be reversible, such as hypothyroidism (an underactive thyroid), and to determine the dementia subtype. One commonly used cognitive test is the Mini-Mental State Examination. The greatest risk factor for developing dementia is aging, however dementia is not a normal part of aging. Many people aged 90 and above show no signs of dementia. Several risk factors for dementia, such as smoking and obesity, are preventable by lifestyle changes. Screening the general older population for the disorder is not seen to affect the outcome.

Dementia is currently the seventh leading cause of death worldwide and has 10 million new cases reported every year (one every ~3 seconds). There is no known cure for dementia. Acetylcholinesterase inhibitors such as donepezil are often used and may be beneficial in mild to moderate disorder. The overall benefit, however, may be minor. There are many measures that can improve the quality of life of people with dementia and their caregivers. Cognitive and behavioral interventions may be appropriate for treating associated symptoms of depression.