I am trying to create an accordion with jQuery from this example

http://docs.jquery.com/UI/Accordion

The markup is the same, i.e.

<div id="accordion">
   <h3><a href="#">First header</a></h3>
   <div>First content</div>
   <h3><a href="#">Second header</a></h3>
   <div>Second content</div>
</div>

And I have script file enqueued correctly, which has the following content:

$ = jQuery;
$(document).ready(function() {
   $("#accordion").accordion();
});

But I get this error in the console output

TypeError: 'undefined' is not a function (evaluating '$("#accordion").accordion()')

The result being all divs are expanded, i.e not clickable.