mental illness n MENTAL DISORDER
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a disorder of one or more of the functions of the mind (such as emotion, perception, memory, or thought), which causes suffering to the patient or others. If the sole problem is that the individual's behaviour as a whole is out of line with society's expectations, then the term 'illness' is not appropriate.
Mental illness should be distinguished from mental retardation, in which an individual has a general failure of development of the normal intellectual capacities. It is broadly divided into psychosis, in which the capacity for appreciating reality is lost, and neurosis, in which insight is retained.
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see under disorder.Medical dictionary. 2011.