(function( window, document, undefined ) {

var xxx = window;

var yyy = document;

    [...] // The main sources are here

     

window.jQuery = window.$ = jQuery;




})(window, window.document);







The same as ....







(function( a, b, undefined ) {

var xxx = a;

var yyy = b;

    [...] // The main sources are here

     

window.jQuery = window.$ = jQuery;




})(window, window.document);




undefined is used to prevent others to declare it as:

var undefined = true;