In Orwell's 1984, what is doublethink?

In George Orwell's dystopian classic 1984, doublethink is the act of holding, simultaneously, two opposite, individually exclusive ideas or opinions and believing in both simultaneously and absolutely. Doublethink requires using logic against logic or suspending disbelief in the contradiction.

The three slogans of the party — "War Is Peace; Freedom Is Slavery; Ignorance Is Strength" — are obvious examples of doublethink. The act of doublethink also occurs in more subtle details throughout the novel.