Dementia is a syndrome associated with many neurodegenerative diseases, characterized by a general decline in cognitive abilities that affects a person's ability to perform everyday activities. This typically involves problems with memory, thinking, behavior, and motor control. Aside from memory impairment and a disruption in thought patterns, the most common symptoms of dementia include emotional problems, difficulties with language, and decreased motivation. The symptoms may be described as occurring in a continuum over several stages. Dementia ultimately has a significant effect on the individual, their caregivers, and their 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 might be caused by the normal aging process.

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 a mild or major neurocognitive disorder with varying degrees of severity and many causative subtypes. The International Classification of Diseases (ICD-11) also classifies 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. It 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, 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. The coexistence of more than one type of dementia is known as mixed dementia.

Many neurocognitive disorders may be caused by another medical condition or disorder, including 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, some leukodystrophies, and neurological disorders such as epilepsy and multiple sclerosis. Some of the neurocognitive deficits may sometimes show improvement with treatment of the causative medical condition.

Diagnosis of dementia 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. Although the greatest risk factor for developing dementia is aging, dementia is not a normal part of the aging process; 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..