I am getting $.ajax is not defined with firebug - maybe missing bracets? any ideas?

<script language="javascript" type="text/javascript">

function get_place(jack)
{
    $.ajax({
       type: "POST",
       url: "fruit.php", 
       beforeSend: function () {
      $("#city").html("<option>Loading ...</option>");
        },
       data: "cartowner="+cartowner,
       success: function(msg){
         $("#city").html(msg);
       }
       });
} 
</script>

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thank you miss dani

thanks alot i spent 4 hours in serchin my error