The Overton Window – the title of Glenn Beck’s new book, a work of fiction which borrows from the idea- is a concept that help drives the work of think tanks like the Commonwealth Foundation.
The Overton Window is named for Joe Overton, founder of the Mackinac Center (our sister group, based in Michigan) – is an idea that politicians choose from policy alternatives within a narrow window, given what is politically viable. It also states that politicians/legislators rarely cause a shift in that window.
However, social movements can shift the window of what is considered possible – which is where the the work of research and advocacy groups comes into play. The Mackinac Center has more detailed description of the Overton Window.