Above the Fold
Referring to the part of the web page that is visible on-screen before the user scrolls down.
The term was originally used with reference to newspapers, where the newspaper would be folded and placed on a news stand. The visible part of the paper (above the fold) is what sells the paper.
In web design, the part of the site that is above the fold has the same function. There must be enough content of interest above the fold to make the user want to dig deeper.