$只是对全局jQuery对象的简短引用.

$ is just a short reference to the global jQuery object.

window.$ === window.jQuery // true

大多数插件作者将美元符号放入自调用方法中，以确保美元符号确实在引用jQuery对象.

Most plugin authors make sure that the dollar sign really is referencing the jQuery object, by putting it into a self-invoking method.

(function( $ ) {
    // $(document)
}( jQuery ));

通过使用jQuery对象作为参数调用该匿名方法，我们可以通过$在该方法内访问它.

By invoking that anonymous method with the jQuery object as argument, we can access it within the method via $.