Learnings.org

the "safe" area of a web page; the uppermost region that will display in a web browser window on the lowest common denominator's computer screen resolution. All of the most important information absolutely must be crammed into this space in order for a viewer to see it as the page launches, because, apparently, most internet users do not know how to scroll.

As in...
"Listen: our product really isn't that great, and people aren't likely to care enough to scroll down the page, so we have to make sure they click on things by accident. That can only happen if we place these buttons and ads above the fold."

Actually, this definition sucks. Let me add my own. You can also share "above the fold" with a friend or share it on Twitter.