Social construct.

[Belacqua]

So what comprises a mountain exists it doesn't matter what we want to call it, but it's a mountain because people agreed that hills over a certain height are mountains and everyone mostly agreed.

If everyone changed their minds it would change what mountains and hills are.

I wonder if it would make sense to differentiate two different types of thing here.

First, social construct as definition of existing thing. So whether the mound is a hill or a mountain, whether Pluto is a planet or something else, depends on definition. The object would exist even if humans were completely unaware of them.

Second, another kind of social construct would be something that only exists because people believe in it. Such things have no physical existence you can point to. They are relationships among people, agreements to act in certain ways, etc. I'm thinking this category could include marriage, other contracts involving promises, etc. Perhaps government. The fact that these things are recorded on paper, or have buildings built to house them, doesn't mean they are concrete objects. The marriage record in city hall is not the marriage, the capitol building is not the government. 

In the case of this second type, if the social construct changed, the agreed-upon thing would cease to exist -- unlike the case of a mountain.