Here's how the World Health Organization defines obsessive-compulsive disorder.
F42 Obsessive-compulsive disorder
The essential feature is recurrent obsessional thoughts or compulsive acts. Obsessional thoughts are ideas, images, or impulses that enter the patient's mind again and again in a stereotyped form. They are almost invariably distressing and the patient often tries, unsuccessfully, to resist them. They are, however, recognized as his or her own thoughts, even though they are involuntary and often repugnant. Compulsive acts or rituals are stereotyped behaviours that are repeated again and again. They are not inherently enjoyable, nor do they result in the completion of inherently useful tasks. Their function is to prevent some objectively unlikely event, often involving harm to or caused by the patient, which he or she fears might otherwise occur. Usually, this behaviour is recognized by the patient as pointless or ineffectual and repeated attempts are made to resist. Anxiety is almost invariably present. If compulsive acts are resisted the anxiety gets worse.
F42.0 Predominantly obsessional thoughts or ruminations
These may take the form of ideas, mental images, or impulses to act, which are nearly always distressing to the subject. Sometimes the ideas are an indecisive, endless consideration of alternatives, associated with an inability to make trivial but necessary decisions in day-to-day living. The relationship between obsessional ruminations and depression is particularly close and a diagnosis of obsessive-compulsive disorder should be preferred only if ruminations arise or persist in the absence of a depressive episode.
F42.1 Predominantly compulsive acts (obsessional rituals)
The majority of compulsive acts are concerned with cleaning (particularly handwashing), repeated checking to ensure that a potentially dangerous situation has not been allowed to develop, or orderliness and tidiness. Underlying the overt behaviour is a fear, usually of danger either to or caused by the patient, and the ritual is an ineffectual or symbolic attempt to avert that danger. (From ICD-10)
Here is some concise general information about OCD from the U.S. National Institute of Mental Health.
Earlier terms for obsessions and compulsions include "scruples" and "scrupulosity." Early physicians considered obsessions and compulsions to be a form of melancholy, a term which meant something different in the past than it does today.
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