In his dystopian novel, Nineteen Eighty-Four, George Orwell described a totalitarian society patterned after Stalinist Russia. Though the novel is most famous for its pervasive surveillance of daily life, Orwell also envisioned that the population could be controlled and manipulated through the alteration of everyday language and thought. The techniques he described were called "newspeak" and "doublethink."
Doublethink was a form of trained, wilfull blindness to the contradictions inherent in a false system of beliefs. In the case of Winston, Orwell's protagonist, it meant being able to work at the Ministry of Information deleting uncomfortable facts from public records, and then believing in the new history which he himself had written.
Over the years since Nineteen Eighty-four was published, the term has grown to be synonymous with relieving cognitive dissonance by simply ignoring the contradiction between two worldviews.
See also: "two plus two make five"