Dementia
Dementia is defined as a deterioration in intellectual abilities, often severe enough to interfere with social and occupational functioning. The impairment involves most aspects of cognition, particularly memory and judgment. Higher cortical functions, such as language, abstract reasoning, and the ability to follow instructions, are generally also impaired.
Dementia is neither synonymous with old age nor a normal part of the ageing process. It may be accompanied by behavioural disturbances, psychosis, mood changes, and delirium (agitation or stupor with clouding of consciousness, often of sudden onset and frequently reversible).
References:
Kaplan & Sadock: Synopsis of Psychiatry, twelfth edition
Kaplan & Sadock: Pocket Handbook of Clinical Psychiatry, fifth edition;
American Psychiatric Association: Diagnostic Manual of Mental Disorders, 1994;
Robert J. Waldinger: Psychiatry for Medical Students, third edition, translated by Anastas Suli.