A mental disorder is an impairment of the mind disrupting normal thinking, feeling, mood, behavior, or social interactions, and accompanied by significant distress or dysfunction.[1][2][3][4] The causes of mental disorders are very complex and vary depending on the particular disorder and the individual. Although the causes of most mental disorders are not fully understood, researchers have identified a variety of biological, psychological, and environmental factors that can contribute to the development or progression of mental disorders.[5] Most mental disorders result in a combination of several different factors rather than just a single factor.[6]
^"Mental, behavioural or neurodevelopmental disorders". International Classification of Diseases for Mortality and Morbidity Statistics, 11th rev. (ICD-11 MMS). World Health Organization. April 2019. Retrieved 2019-10-30. Mental, behavioural and neurodevelopmental disorders are syndromes characterized by clinically significant disturbance in an individual's cognition, emotional regulation, or behaviour that reflects a dysfunction in the psychological, biological, or developmental processes that underlie mental and behavioural functioning. These disturbances are usually associated with distress or impairment in personal, family, social, educational, occupational, or other important areas of functioning.
^Webster's Third New International Dictionary, (Springfield, MA: Merriam-Webster, 1961, rev. 2016), ("mental illness noun, variants: or mental disorder or less commonly mental disease, Definition of mental illness: any of a broad range of medical conditions (such as major depression, schizophrenia, obsessive compulsive disorder, or panic disorder) that are marked primarily by sufficient disorganization of personality, mind, or emotions to impair normal psychological functioning and cause marked distress or disability and that are typically associated with a disruption in typical thinking, feeling, mood, behavior, interpersonal interactions, or daily functioning").
^American Heritage Dictionary of the English Language, 5th ed. (Boston: Houghton Mifflin Harcourt, 2011, rev. 2018), ("mental disorder, n. - Any of various disorders, such as schizophrenia, bipolar disorder, or autism spectrum disorder, characterized by a distressing or disabling impairment of an individual's cognitive, emotional, or social functioning.")
^Oxford English Dictionary, 3rd ed. (Oxford, UK: Oxford University Press, September 2001), ("II. Senses relating to the mind in an unhealthy or abnormal state. 5. a. Designating a temporary or permanent impairment of the mind due to inherited defect, injury, illness, or environment, usually needing special care or rehabilitation. Esp. in mental breakdown, mental deficiency, mental disease, mental disorder, mental incapacity, mental retardation, etc.; see also mental illness n. at Compounds. ... mental illness n. a condition which causes serious abnormality in a person's thinking or behaviour, esp. one requiring special care or treatment; a psychiatric illness. Now somewhat dated, and sometimes avoided as being potentially offensive.").