The
bottom half of a
newspaper or
Web page.
From newspaper journalism: Information located on the half of the paper below the line where the newspaper is folded can't be seen in a stack or in a vending rack. Headlines, photos, and articles above the fold are designed to attract the reader's attention and sell the paper.
In Web design, any information on a Web page not visible on a computer screen without use of the vertical scroll bar is considered below the fold. Designers assume that users new to the Web may not understand page layout or navigation, thus info below the fold may be ignored.