A thoughtcrime is an occurrence or instance of not allowed thoughts.
Freedom of expression
Laws against thoughtcrimes are difficult to enforce without telepathy. More effective are laws limiting freedom of expression of thoughts. Crimes against such laws may also be referred to as "thoughtcrimes", although this is not strictly correct, if these laws only limit expressing thoughts, but not the thoughts themselves.
The term was popularized by the dystopian novel Nineteen Eighty-Four by George Orwell, wherein thoughtcrime is the criminal act of holding unspoken beliefs or doubts that oppose or question the ruling party. In the book, the government attempts to control not only the speech and actions, but also the thoughts of its subjects. To entertain unacceptable thoughts is known as "crimethink" in Newspeak, the ideologically purified dialect of the party.
"Crimestop" is a way to avoid crimethink by immediately purging dangerous thoughts from the mind.
The "Thought Police" (thinkpol in Newspeak) are the secret police of the novel. It is their job to uncover and punish thoughtcrime. The "Thought Police" use surveillance and psychological monitoring to find and eliminate members of society who challenge the party's authority and ideology.
Technology played a significant part in the detection of thoughtcrime in Nineteen Eighty-Four—with the ubiquitous telescreens which could inform the government, misinform and monitor the population. The citizens of Oceania are watched by the "Thought Police" through the telescreens. Every movement, reflex, facial expression, and reaction is measured by this system.
The "Thought Police" of the novel was partially based on methods used by the totalitarian states and ideologies of the 20th century. The term "Thought Police", by extension, has come to refer to real or perceived enforcement of ideological correctness.
Monitoring information access
Surveillance organizations may used methods such as monitoring what information is accessed in order to search for individuals with possibly dangerous thoughts. For example, library loans may be monitored, with certain books being on lists, and individuals loaning these books may be considered to possibly have dangerous thoughts that may lead to illegal actions.
Religious laws, often assuming a god or gods that may perceive and judge thoughts, may prohibit certain thoughts.
Individuals may feel guilt if thinking certain thoughts and especially thoughts considered wrong by their culture. Such thoughts may possible be considered thoughtcrimes in this particular culture, even if there are no formal laws against such thoughts. Examples in modern societies may include thinking not politically correct thoughts or thinking thoughts contrary to Holocaustianity.