html - What is the meaning of the "Use of undefined constant" notice in PHP?

Notice: Use of undefined constant username - assumed 'username' in
/home/content/04/7195304/html/header.php on line 54

I get this when writing things like $_COOKIE[username] or $_POST[username].

Edit

So I've been playing around with the code, putting quotes in my POST, COOKIE, and GET's.. I still get the same thing!