Same Origin Policy
When loading a document from one origin, a script loaded from a different origin cannot get or set certain predefined properties of certain browser and HTML objects in a window or frame.
Origin is defined as protocol://host, where host may include optional parts of URL including :port, part of an URL.
Any applets in the document are also subject to origin checks when calling JavaScript.
The same origin policy is the default policy since Netscape 2.
Properties subject to origin check