Why did I get this error: 0x800a1391 - Microsoft JScript runtime error: '$' is undefined. ??

Hi, I am uisng VS2012 to build web form project, there are some Javascript codes in .aspx page, no problem when compile the page, but when I run it, I got this error: 0x800a1391 - Microsoft JScript runtime error: '$' is undefined.
Do I need to import any jquery scripts? or javascript core library? or do I miss any namespace related to Javascript? thaks for any help,
The javascript codes are highlighted:
<script language="javascript" type="text/javascript">
          $(document).ready(function () {             //the codes are highlighted from here.
                      $('.ExpandCollapseStyle').click(function () {              &n ...