A term coined by George Orwell in 1949 for his novel "1984" to describe illogical or deliberately perverse thought which serves to distort or reverse the truth to make unpopular or dangerous ideas more acceptable.
For example: "War is peace" to persuade the masses that waging a war is the only path to a lasting peace; or "Freedom is slavery" to convince the populace to welcome an authoritarian dictatorship; or "Trump is smart" to indoctrinate disciples to reject the evidence they can clearly see to bind them ever closer to the cult of the Leader.