Are you referring to HTTP access control (CORS)? If yes!
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.
For security reasons, browsers restrict cross-origin HTTP requests initiated from within scripts. For example, XMLHttpRequest follows the same-origin policy. So, a web application using XMLHttpRequestcould only make HTTP requests to its own domain. To improve web applications, developers asked browser vendors to allow XMLHttpRequest to make cross-domain requests.
The W3C Web Applications Working Group recommends the new Cross-Origin Resource Sharing(CORS) mechanism. CORS gives web servers cross-domain access controls, which enable secure cross-domain data transfers. Modern browsers use CORS in an API container - such as XMLHttpRequest - to mitigate risks of cross-origin HTTP requests.
This article is for web administrators, server developers and front-end developers. Modern browsers handle the client-side components of cross-origin sharing, including headers and policy enforcement. But this new standard means servers have to handle new request and response headers. Another article for server developers discussing cross-origin sharing from a server perspective (with PHP code snippets) is supplementary reading.
This cross-origin sharing standard is used to enable cross-site HTTP requests for: