The Overton window is the range of policies politically acceptable to the mainstream population at a given time. It is also known as the window of discourse. The term is named after Joseph P. Overton. According to Overton, the window frames the range of policies that a politician can recommend without appearing too extreme to gain or keep public office given the climate of public opinion at that time.
"The smart way to keep people passive and obedient is to strictly limit the spectrum of acceptable opinion, but allow very lively debate within that spectrum—even encourage the more critical and dissident views. That gives people the sense that there's free thinking going on, while all the time the presuppositions of the system are being reinforced by the limits put on the range of the debate." - Noam Chomsky