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.
You will usually hear the Overton window used in relation to the centre ground, and whether a certain party has managed to shift it to the left or right. For example, some argue that Ukip has shifted the Overton window to the right, by making room for the main parties to formulate harsher policies on immigration than the UK has previously experienced.
Its originator was Joseph P Overton, a former vice president of the Mackinac Center for Public Policy. He developed the theory in the mid-Nineties.