ReferenceError: não está definido

estou a usar um widget WordPress Twitter baseado em jQuery e receber o erro " ReferenceError: _ is not defined".

não sei como declarar a variável"_".

Aqui está o elemento:

<script type="text/javascript">
    <!--//--><![CDATA[//><!--

        function twitterCallback2(twitters) {

            var statusHTML = [];
            for (var i=0; i<twitters.length; i++){
                var username = twitters[i].user.screen_name;
                var status = twitters[i].text.replace(/((https?|s?ftp|ssh)\:\/\/[^"\s\<\>]*[^.,;'">\:\s\<\>\)\]\!])/g, function(url) {
                    return '<a href="'+url+'">'+url+'</a>';
                }).replace(/\B@([_a-z0-9]+)/ig, function(reply) {
                    return  reply.charAt(0)+'<a href="http://twitter.com/'+reply.substring(1)+'">'+reply.substring(1)+'</a>';
                });
                statusHTML.push( '\
                <li class="tweet">\
                    <span class="content">'+status+'\
                    <a style="font-size:85%" class="time" href="http://twitter.com/'+username+'/statuses/'+twitters[i].id_str+'">('+relative_time(twitters[i].created_at)+')</a></span>\
                <div class="clearfix"></div></li>' );
          }
          document.getElementById('twitter_update_list_<?php echo $unique_id; ?>').innerHTML = statusHTML.join('');

          var template = '\
            <span class="author">\
                <img src="<%= user.profile_image_url %>">&nbsp;\
                <a class="username" href="http://twitter.com/<%= user.screen_name %>">\
                    <strong><%= user.screen_name %></strong>\
                </a>\
            </span>';
          jQuery( _.template( template, { user: twitters[0].user } )).insertAfter('.bizsteam_twitter ul');
        }

        function relative_time(time_value) {
          var values = time_value.split( " " );
          time_value = values[1] + " " + values[2] + ", " + values[5] + " " + values[3];
          var parsed_date = Date.parse(time_value);
          var relative_to = (arguments.length > 1) ? arguments[1] : new Date();
          var delta = parseInt((relative_to.getTime() - parsed_date) / 1000);
          delta = delta + (relative_to.getTimezoneOffset() * 60);

          if (delta < 60) {
            return 'less than a minute ago';
          } else if(delta < 120) {
            return 'about a minute ago';
          } else if(delta < (60*60)) {
            return (parseInt(delta / 60)).toString() + ' minutes ago';
          } else if(delta < (120*60)) {
            return 'about an hour ago';
          } else if(delta < (24*60*60)) {
            return 'about ' + (parseInt(delta / 3600)).toString() + ' hours ago';
          } else if(delta < (48*60*60)) {
            return '1 day ago';
          } else {
            return (parseInt(delta / 86400)).toString() + ' days ago';
          }
        }
    //-->!]]>

    </script>
    <?php
        if($exclude_replies != ''){ $exclude_replies_str = '&amp;exclude_replies='.$exclude_replies; } else { $exclude_replies_str = ''; }
    ?>
    <script type="text/javascript" src="http://api.twitter.com/1/statuses/user_timeline/<?php echo $twitter_username; ?>.json?callback=twitterCallback2&amp;count=<?php echo $twitter_count; ?>&amp;include_rts=t<?php echo $exclude_replies_str; ?>"></script>

Firebug afirma que a linha de código com o erro é:

jQuery( _.template( template, { user: twitters[0].user } )).insertAfter('.bizsteam_twitter ul');

qualquer ajuda seria muito apreciada