The upper half of a newspaper when folded which receives greater attention because of its visibility, or in web design and advertising, that portion of a page that is initially visible without scrolling. In that case it is somewhat fluid because of varying screen dimensions that must be anticipated.
Generally content and advertising above the fold will receive greater attention and higher click through rates that result in better sales conversions.
Advertising rates therefore command higher rates if they are above the fold.
Contrast with Below the Fold.