Uncaught TypeError: Can not read property 'post' of undefined

My code so far:

<p><script language="javascript">
function funcion(){
    var a ="field_1";
    var b ="field_2";
    var c ="field_3";
    var d ="field_4";
    var e ="field_5";
    var f ="field_6";
    var g ="field_7";
    var aa = document.getElementById(a).value;
    var bb = document.getElementById(b).value;
    var cc = document.getElementById(c).value;
    var dd = document.getElementById(d).value;
    var ee = document.getElementById(e).value;
    var ff = document.getElementById(f).value;
    var gg = document.getElementById(g).value;
    $.post("Aqui va mi URL", { dni: aa, proc: bb, firma: cc, exp: dd, us: ee, "11": ff, "10": gg }, <br>function(data){
    $("#principal").html(data);
    });
}
window.onload = funcion;

So far the JavaScript and the HTML are simple. There are 7 normal and ordinary fields.

What I'm trying to do is that when you send the data to a page, it sends it back again. That is, page 1 sends data to page 2, and page 2 enters a database and automatically sends them to page 3

I get the following error.

  

Uncaught TypeError: Can not read property 'post' of undefined       at work       at window.onload

Does anyone have any idea why?

That same code I used before, just modify one or two things. Although I do not know what this is "$("#principal").html(data);" . I put the form of that ID name just in case, maybe it's wrong.

Thanks for the inconvenience.