Psychological Operations (PSYOP, PSYOPS) are techniques used by military and police forces to influence a target audience’s value systems, belief systems, emotions, motives, reasoning, and behavior. Target audiences can be governments, organizations, groups, and individuals, and are used in order to induce confessions, or reinforce attitudes and behaviors favorable to the originator’s objectives. These are sometimes combined with black operations or false flag tactics.
This concept has been used by military institutions throughout history, but it is only since the twentieth century that it has been accorded the organizational and professional status it enjoys now.
The word is commonly used by governments, such as the government of the United States, who do not wish to use the term propaganda or brainwashing to refer to their own work. The word propaganda has very negative connotations, and by calling it psychological operations instead, more sophisticated methods of psychological manipulation are accurately incorporated by the terminology. This euphemism for mind control is ironically an example of psychological operations — i.e. using psychological techniques to persuade [manipulate] a large number of people to support something that they wouldn’t normally support.
Psychological Operations (United States)
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The purpose of United States psychological operations (PSYOP) is to induce or reinforce attitudes and behaviors favorable to U.S. objectives. It can be used at the strategic, operational, also known as Psychological warfare, level or at the tactical level. Strategic psychological operations are done by government agencies other than the military, except, if delegated to the military, in major wars and at the level of theaters of operations.
Psychological operations are a subset of information operations, defined as:
Planned operations to convey selected information and indicators to foreign audiences to influence their emotions, motives, objective reasoning, and ultimately the behavior of foreign governments, organizations, groups, and individuals. The purpose of psychological operations is to induce or reinforce foreign attitudes and behavior favorable to the originator’s objectives. Also called PSYOP. 
PSYOP is a subset of information operations:
The integrated employment of the core capabilities of electronic warfare, computer network operations, psychological operations, military deception, and operations security, in concert with specified supporting and related capabilities, to influence, disrupt, corrupt or usurp adversarial human and automated decision making while protecting our own. Also called IO.
Psychological operations have very diverse applications, ranging from the relatively unprovocative, such as the “mine awareness” campaign in Bosnia and Kosovo, designed to curtail civilian deaths due to land mines, to programs that have incurred criticism from various groups. Information operations includes psychological operations, but also such functions as denying the enemy of the use of their own public communications or offensive psychological operations.