jquery script not working in my server

[php_begins]

I wrote this j query script which works fine in the jsfiddle.net but does not work in the server at www.cs.txstate.edu/~sr1388.

In firebug it says $ not defined. Am i Missing any file?

<script type="text/javascript">
$(document).ready(function() {  
  $("#credit_pay").hide();
  $("#downpayment_method").change(function() {
      if ($("#downpayment_method").val()=='credit_card') {
       $("#credit_pay").show("fast");
      } else { 
       $("#credit_pay").hide("fast");
    };
      if ($("#downpayment_method").val()=='e_check') {
       $("#e_pay").show("fast");
      } else { 
       $("#e_pay").hide("fast");
    };
  });
});
</script>