Overton Window
Refers to the range of political policies that are considered acceptable at a particular moment in time. Anyone proposing policies outside the Overton Window are considered “extreme.”
Examples
The idea of freeing slaves was far outside the Overton Window in the early 1800s. Abolitionists were viewed as crazy. Gradually, the window shifted on slavery and the idea of abolition became more and more mainstream, until being in favor of continuing slavery started to drift outside the window.