The Overton Window is a theory from political and social science, referring the area of behaviour or beliefs that are considered acceptable in a society. It is named after Joseph Overton, who worked for a right wing think tank in the US in the 1990s. The concept has been understood for centuries but the term Overton Window is much more recent.
The concept is that the window can be moved, making what is deemed to be acceptable in the society further to the previous extremes. The most common way to shift the window is for some proponents to take an extreme position far outside the norm thus making things just outside the window, though less extreme, seem more acceptable and so shifting to a new “middle ground”.
The Overton Window shifting has been evident to anyone following the Brexit saga. Very few proponents of Brexit prior to the referendum advocated leaving without any form of deal at all. Over the last 3 years, constant calls of 'No Deal being better than a Bad Deal' has made idea of leaving with no deal far more acceptable and seemingly normal.