【初学者】大家帮我看看这是什么原因

<script src="js/jquery-ui-1.9.0.custom.min.js" type="text/javascript"></script>

    <script type="text/javascript">

        function a() {

            var x = document.getElementsByTagName("td");

            if (x.length == 0) {

                alert("");

                return false;

            }

            var y = x[0];

            y.parentNode.removeChild(y);

            alert(x.length);

        }

    </script>

<div>

        <table>

            <tr>

                <td>

                    1

                </td>

                <td>

                    2

                </td>

                <td>

                    3

                </td>

                <td>

                    4

                </td>

            </tr>

        </table>

        <input type="button" onclick="a()" />

    </div>

点击一次删除一个<td>
效果已经实现，但是“f12”中有个错误:
Uncaught ReferenceError: jQuery is not defined
(anonymous function)

到底是什么原因，新手在学js,请大侠们帮我解释下