The Overton Window describes the range of ideas that are tolerated in public discourse. Everything outside of it is considered "extreme", unplausible or fanatical, hallucinatory. Unthinkable. Irrational. Like flying harpies with horns who are surreptitiously responsible for anything from obstetric complications to spoiled food.
This window can be changed and moved depending on political climate, context, dissemination of an important discovery, and the kinds of things important figures are saying, repeatedly. These forces tug the window along a spectrum and suddenly things that were once unthinkable are plausible, sensible, popular in discussion-- finally they can become policy.
Adam's first wife, Lilith, who proposed to God being equal to your spouse, was considered in her time ridiculous; the unthinkable: A demon. Her demand for equality went far outside the window, resulting in immediate ejection from paradise. She was a phobic figure who lived in what could only be hell.