JQuery Ajax data.d含义

$.ajax({

            url: 'EmailExist.action',

            data: '{email:"' + email + '"}',

            type: 'post',

            dataType: 'json',

            contentType: 'application/json; charset=utf8',

            cache: false,

            success: function(data) {

                if (data.d) {

                    setTimeout(function() {

                        //$('#ctl00_holderLeft_txt_email').val('');

                        $('#tip_email').removeClass('validation-valid');

                        $('#tip_email').addClass('validation-error');

                        $('#tip_email').html('邮箱<strong>"' + email + '</strong>"已存在，请使用其他邮箱');

                    }, 10);

                }

                //else

                    //$('#tip_email').html("邮箱输入成功");

            },

            error: function(xhr) {

                $('#tip_email').html(xhr.responseText);

            }

        });

Ajax中有个data.d，请问下各位大牛们这个data.d是个什么含义？
------解决方案--------------------
这个d是看