What is this math symbol called? My professors use a symbol $$x_0$$ and they pronounce it as x not or x nod, I am not sure what the exact name is because they have thick accents. I have tried looking this up on the Internet but I could not find an answer. Does anyone know what this is called?

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.
