Word for how society gradually and in steps moves to accept an idea that was once beyond even consideration [closed]

Question

I'm trying to remember a word I once knew. 

What is the word that recognizes how society goes from, first, totally not accepting something to, second, having it become in the realm of possibility as something to consider to, third, having it accepted by the majority?

For example, 30 years ago gay marriage wasn't really even on the radar. Then, it became something that was controversial, but still not accepted by most. Finally, it became accepted by the majority.

There is some word or concept or theory the definition of which is how society gradually and in steps moves to accept an idea that was once beyond even consideration.  I cannot think of this word, but I know it exists. 

Answer 1

I'm made to think of the Overton window, but that doesn't inherently involve change.

I originally posted this as a comment because I didn't think it was a satisfactory answer, but apparently "Overton window" is indeed the term that the asker was trying to remember, so I am adding this as a formal answer.

The linked Wikipedia article says (bold emphasis mine):

The Overton window is the range of policies politically acceptable to the mainstream population at a given time.[1] It is also known as the window of discourse.

The term is named after American policy analyst Joseph Overton, who stated that an idea's political viability depends mainly on whether it falls within this range, rather than on politicians' individual preferences.[2][3] According to Overton, the window frames the range of policies that a politician can recommend without appearing too extreme to gain or keep public office given the climate of public opinion at that time.

[...]

Overton described a spectrum from "more free" to "less free" with regard to government intervention, oriented vertically on an axis, to avoid comparison with the left/right political spectrum.[8] As the spectrum moves or expands, an idea at a given location may become more or less politically acceptable. After Overton's death, his Mackinac Center for Public Policy colleague Joseph Lehman further developed the idea and named it after Overton.[9]

[...]

— "Overton window," Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Overton_window&oldid=1077856689, copied under CC BY-SA 3.0 licence

Answer 2

Sounds like the nature of man; progressive, towards an idea that was once not accepted by many. Progressive is the word I would use. Example a) flying to the moon is something that was once not conceivable. For instance, flying to the moon in the medieval ages was less a reasonable thought. Only in due time has mankind made inventions that brought us closer to this possibility. From the dark ages we transversed into the renaissance and rebirth era, then into the industrial revolution and in the last 100 years we have made our greatest strides toward technological advancements. Orbiting Saturn, and understanding the makeup of blackholes was not even a thought in the Middle Ages as no one knew they existed.