What mean by "e" in different functions?

What mean by "e" in different functions?

Hello I am new to jquery. Recently I read some beginner tutorial and got follow code: 

  1. <script type="text/javascript">
     function showMsg(e) {
     $(e.target).attr('disabled', true);
     if ($('#msg').html().length == 0) {
     $('#msg').html('



    <h1>Hello</h1>');
     }
     $('#msg').fadeIn();
     setTimeout(function(){
     $('#msg').fadeOut();
     $(e.target).attr('disabled', false);
     }, 3000);
     }

     $(document).ready(function(e){
     $('#btn').click(showMsg);
     });
     











    </script>
I have some experience in ASP and PHP but seldom reach client side language like javascript. I fail to understand what mean by the "e" in showMsg(e) and function(e). I guess e = event. But I am not sure what event it refer to since the event not defined anywhere in code. Is it default value? Can someone fill me the details? Thank for any input!