"Doublethink means the power of holding two contradictory beliefs in one's mind simultaneously, and accepting both of them. ... To tell deliberate lies while genuinely believing in them, to forget any fact that has become inconvenient, and then, when it becomes necessary again, to draw it back from oblivion for just so long as it is needed, to deny the existence of objective reality and all the while to take account of the reality which one denies—all this is indispensably necessary. Even in using the word doublethink it is necessary to exercise doublethink. For by using the word one admits that one is tampering with reality; by a fresh act of doublethink one erases this knowledge; and so on indefinitely, with the lie always one leap ahead of the truth."
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"