What Is a Bubble?

In 1720, in French, there was a stock market crash in Europe. And they referred to it as a bubble. So I think they were bringing in a metaphor that's a little bit unfortunate. It sounds like when you're blowing up a bubble, a soap bubble,. You're trying to get it as big as you can. But it has a limit. It can only get so big and then pop. In farmers' Nobel Lecture, he defined the word bubble as being a strong run-up in prices followed by a predictable strong decline. How do you define the word bubble? Here I go again.