What is this math symbol called? [closed]

Solution 1:

It's probably x-naught, synonym for "x sub zero", it's used when you refer to an starting point for variable $x$, for example in physics, if you have a particle moving on the $x$ axis, you will always find $x_0$ for the initial position.

Solution 2:

They actually call it x-naught. I believe it comes from British English. Kind of like how the Canadians call the letter z "zed". All it means is "x sub zero", just another way of saying the same thing. It does flow better though, I think. "sub zero" just takes so much more work to say. I do think "naught" and "not" have similar meaning though - the absence of something, some value or quality. Im sure there is a linguistic connection.