Uncaught Reference error $ is not defined

Code:
function checkavailableNames()

{        

   var username=document.registration_form.indiv_reg_loginname.value

   if(username == '')

   {

      document.getElementById('indiv_username_avail_check')[removed]="Required field cannot be left blank";

      document.registration_form.indiv_reg_loginname.focus();

      return false;

    }

    else

    {    

    $.ajax({

    type: 'POST',

    dataType: 'json',

    url: base_url + 'register/checkIndivUsernameAvail/' + Math.random(),

    data: 'username=' + username,

    success: function(response)

        {            

       var availcontent    = response.result;

       if(availcontent.length > 0)

       {

            document.getElementById('indiv_username_avail_check')[removed]="User name is not available";

        return false;

       }

       else

       {

        document.getElementById('indiv_username_avail_check')[removed]="User name is available";

        return true;

       }                                    

    }                

    });            

    }    

}