Uncaught ReferenceError: $ is not defined

Olá, como faço para resolver esse erro no meu código? Uncaught ReferenceError: $ is not defined

$(document).ready(function(){
        // Function sortable
        $( function() {
            $( "#Essencial, #Importante, #Superfluo" ).sortable({

                items: "li:not(.groupLi)",
                connectWith: ".connectGroup",
                cursor: 'move',
                cursorAt: { left: 20 },
                tolerance: 'pointer',
                revert: 'invalid',
                placeholder: 'span2 well placeholder tile',

                receive: function(event, ui) {

                   $('li.groupLi', ui.sender).hide();

                   if($('li:not(.groupLi)', ui.sender).length == 0){
                       $('li.groupLi', ui.sender).show();

                   } else {

                       if ($('#Essencial').children('li').length > 0){
                             $('#Essencial li:contains(Arraste um item)').hide();
                       }

                       if ($('#Importante').children('li').length > 0){
                             $('#Importante li:contains(Arraste um item)').hide();
                       }

                       if ($('#Superfluo').children('li').length > 0){
                           $('#Superfluo li:contains(Arraste um item)').hide();
                       }
                   } 

                    var urlData =  ui.sender[0].id + '/' +  this.id + '/' +  ui.item[0].id;

                    $.ajax( "despesasNomeId/" + urlData)
                        .success(function( urlData ) {
                        })
                        .error(function( urlData ) {
                        });
                },

                update : function( event, ui){

                    //Contagem dos itens
                    var countEss = $('#Essencial li').not(".ui-sortable-helper").not(".groupLi").length;
                    var countImp = $('#Importante li').not(".ui-sortable-helper").not(".groupLi").length;
                    var countSup = $('#Superfluo li').not(".ui-sortable-helper").not(".groupLi").length;

                    $('#countEss').html(countEss);
                    $('#countImp').html(countImp);
                    $('#countSup').html(countSup);
            },
                stop : function( event, ui){},

            }).disableSelection();
       } );

    })