Understanding concept of plugin - $ / jQuery is not defined

Understanding concept of plugin - $ / jQuery is not defined

I wrote one plugin with following syntax:
(function($){
$.fn.samplePlugin = function() {
return this.each(function() {
//My logic here
});
};
})(jQuery);
Then i called on load as
$(document).ready(function(){
$('#sample').samplePlugin();
});
Now i have these two errors in my console:
ReferenceError: jQuery is not defined
ReferenceError: $ is not defined
Can you please tell me what i'm missing and what should be the flow of
usage of $ annotation when u create or include plugins?
Thanks,