Cross-Origin Resource Sharing
A resource makes a cross-origin HTTP request when it requests a resource from a different domain than the one which the first resource itself serves. For example, an HTML page served from http://domain-a.com makes an <img> src request for http://domain-b.com/image.jpg. Many pages on the web today load resources like CSS stylesheets, images and scripts from separate domains.
https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS
http://www.w3.org/TR/cors/