Brainwashing (also known as thought reform or reeducation) consists of any effort aimed at instilling certain attitudes and beliefs in a person — beliefs sometimes unwelcome or in conflict with the person's prior beliefs and knowledge, in order to affect that individual's value system and subsequent thought-patterns and behaviors. In 1987, the American Psychological Association (APA) Board of Social and Ethical Responsibility for Psychology (BSERP) provisionally declined to endorse one particular approach to brainwashing as "lack[ing] the scientific rigor and evenhanded critical approach necessary for APA imprimatur". The debate amongst APA members on this subject continues.

The English words "re-educate" and "re-education", which the Oxford English Dictionary attests in general senses from 1808, began in the 1940s to express specifically political connotations. George Orwell mentioned in Animal Farm (1945) "the Wild Comrades' Re-education Committee (the object of this was to tame the rats and rabbits)"; and Arthur Koestler in The Age of Longing (1951) wrote of "revolutionary vigilance,.. and discipline, and re-education camps".

The term "brainwashing" first came into use in the English language in the 1950s. The OED records its earliest known English-language usage of "brain-washing" by E. Hunter in New Leader on 7 October 1950. John D. Marks claimed that Edward Hunter was "later revealed" to have worked undercover for the Central Intelligence Agency (CIA).

Earlier forms of coercive persuasion occurred for example during the Inquisition and in the course of show trials against "enemies of the state" in the Soviet Union; but no specific term emerged until the methodologies of these earlier movements became systematized during the early decades of the People's Republic of China for use in struggles against internal class enemies and foreign invaders. Until that time, presentations of the phenomenon described only concrete specific techniques.

The term xǐ năo (洗腦, the Chinese term literally translated as "to wash the brain") originally referred to methodologies of coercive persuasion used in the "reconstruction" (改造 gǎi zào) of the so-called feudal (封建 fēng jiàn) thought-patterns of Chinese citizens raised under pre-revolutionary régimes; the term punned on the Taoist custom of "cleansing/washing the heart" (洗心 xǐ xīn) prior to conducting certain ceremonies or entering certain holy places, and in Chinese, the word "心" xīn also refers to the soul or the mind, contrasting with the brain. The term first came into general use in the United States in the 1950s during the Korean War (1950–1953) to describe those same methods as applied by the Chinese communists to attempt deep and permanent behavioral changes in foreign prisoners, and especially during the Korean War to disrupt the ability of captured United Nations troops to effectively organize and resist their imprisonment.

The word brainwashing consequently came into use in the United States of America to explain why, unlike in earlier wars, a relatively high percentage of American GIs defected to the enemy side after becoming prisoners-of-war in Korea. Later analysis determined that some of the primary methodologies employed on them during their imprisonment included sleep-deprivation and other intense psychological manipulations designed to break down the autonomy of individuals. American alarm at the new phenomenon of substantial numbers of U.S. troops switching their allegiance to support foreign Communists lessened after the repatriation of prisoners, when it emerged that few of them retained allegiance to the Marxist and "anti-American" doctrines inculcated during their incarcerations. When rigid control of information ceased and the former prisoners' "natural" cultural methods of reality-testing could resume functioning, the superimposed values and judgments rapidly decreased.

Although the use of brainwashing on United Nations prisoners during the Korean War produced some propaganda-benefits to the forces opposing the United Nations, its main utility to the Chinese lay in the fact that it significantly increased the maximum number of prisoners that one guard could control, thus freeing other Chinese soldiers for front-line battlefield duties.

After the Korean War the term "brainwashing" came to apply to other methods of coercive persuasion and even to the effective use of ordinary propaganda and indoctrination. Formal discourses of the Chinese Communist Party came to prefer the more clinical-sounding term sī xǐang gǎi zào 思想改造 ("thought reform"). Metaphorical uses of "brainwashing" extended as far as the memes of fashion-following.