https://www.newstatesman.com/politics/2015/04/what-overton-window
Extract
The Overton window is a political theory that refers to the range (or window) of policies that the public will accept.
The idea is that any policy falling outside the Overton window is out of step with public opinion and the current political climate, and formulated to try and shift the Overton window in a different direction, or to expand it to be wider.
How my heart sings 