The Overton Window

Overton described a method for moving that window, thereby including previously excluded ideas, while excluding previously acceptable ideas. The technique relies on people promoting ideas even less acceptable than the previous "outer fringe" ideas. That makes those old fringe ideas look less extreme, and thereby acceptable. The idea is that priming the public with fringe ideas intended to be and remain unacceptable, will make the real target ideas seem more acceptable by comparison.
The degrees of acceptance of public ideas can be described roughly as:

• Unthinkable
• Radical
• Acceptable
• Sensible
• Popular
• Policy

The Overton Window is a means of visualizing which ideas define that range of acceptance by where they fall in it, and adding new ideas that can push the old ideas towards acceptance merely by making the limits more extreme.