[jQuery] using doc.ready in an external .js file

[jQuery] using doc.ready in an external .js file


Here's what I'm not getting about jQuery from a novice point of view.
Let's say I have this:
    <script type="text/javascript">
        $(document).ready(function() {
            $("#switcherBtn").click(function(){
                $("#target").toggleClass("nodisplay");
            });
        });
    </script>
I have a function to toggle the display of a certain div. This div is
on virtually all my pages, though, so instead of putting this in the
header of each page, I want to put it in a global include file, for
example main.js.
I don't understand why copying and pasting
        $(document).ready(function() {
            $("#switcherBtn").click(function(){
                $("#target").toggleClass("nodisplay");
            });
        });
into main.js throws this error in Firebug:
$ is not defined
main.js()()main.js (line 1)
$(document).ready(function(){
Any input?