Uncaught TypeError: Cannot read property 'ajax' of undefined When I'm trying to send data to url

I have created a form to get feedback from user, I'm simply trying to send form data to url. but I'm getting this error. Uncaught TypeError: Cannot read property 'ajax' of undefined

thank you in advance.

 function sendData(){

            $.ajax({
                url: "www.yashkjhsah.php",

                type: "POST",

                async: true,

                data: $(".contacts_form").serialize(),

                dataType: "html",

                success: function(data) 
                 {
                alert(data);

                if(data!="Error in Insert Query!")
                {
                    alert("Thank you for Enquiry we will send answer in your Mail.");
                }
                else
                {
                alert("Error while saving the data");
                }
                 }
             });

}