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?

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.
