$ document .ready function { $ is not defined 这个提示错误为什么

<script>
	$(document).ready(function(){
		$("#demoForm").validate(
		{
			rules:{
				username:{
					required:true,
					minlength:2,
					maxlength:10
				},
				password:{
					required:true,
					minlength:2,
					maxlength:16
				}
			},
			messages:{
				username:{
					required:"用户名必须填写",
					minlength:"用户名最小为2位",
					maxlength:"用户名最大为10位"
				},
				password:{
					required:"必须填写密码",
					minlength:"密码最小为2位",
					maxlength:"密码最大为16位"
				}

			}
		});

	});
</script>