CORS

An abbreviation of "Cross-Origin Resource Sharing"
: a browser mechanism which enables controlled acess to resouces located outside of a given domain.
It extends and adds flexibility to "the same-origin policy."
However, it also provides potential for cross-domain attacks.

The SOP dictates that when your browser loads a scrip from a particular (sub)domain, that the script can only make requests to the (sub)domain that it originated from.