The Overton Window
Rebecca Blood has a good blog post on the Overton Window, an important concept I think we should all be familiar with. From the Wikipedia article, the concept:
"describes a "window" in the range of public reactions to ideas in public discourse, in a spectrum of all possible options on an issue. 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. Delivering rhetoric to define the window provides a plan of action to make more acceptable to the public some ideas by priming them with other ideas allowed to remain unacceptable, but which make the real target ideas seem more acceptable by comparison."