From the link:
[In George Orwell’s novel, 1984,] "doublethink" is one of the three most important principles of INGSOC (along with the mutability of the past, and Newspeak).
Doublethink refers to the ability to hold two contradictory thoughts in one’s head. ...
[The ideas] are a result of political indoctrination. Meaning, they are only present because [political] force convinced the thinker that both are true. ...
[The term is defined as] Winston Smith is thinking about doublethink. The narrator states:
To know and not to know, to be conscious of complete truthfulness while telling carefully constructed lies, to hold simultaneously two opinions which cancelled out, knowing them to be contradictory and believing in both of them, to use logic against logic, to repudiate morality while laying claim to it, to believe that democracy was impossible and that the Party was the guardian of democracy, to forget whatever it was necessary to forget, then to draw it back into memory again at the moment when it was needed, and then promptly to forget it again: and above all, to apply the same process to the process itself.
Link: https://bookanalysis.com/1984/doublethink/