Terry Moore: Why is 'x' the unknown?

I just LOVE the history of mathematics. I find it so fascinating to learn about the stories behind what we do, the theorems that we study and mathematics who discovered them.

A question that is commonly asked - and, I for one didn't know the answer to - why do we use x to represent the unknown.

Terry Moore explains why in his TedTalk taking us on a tour back to Arabic mathematics of the first few centuries CE to mathematics spread in Europe.

Enjoy :)