“Above the fold” refers to the issue of content visualization on an digital interface without having to use vertical scrolling.
"Above the fold" is an issue because content below the fold may not be seen. Therefore, it is not recommended to have a call to action or validation button below the fold.
Unfortunately for website designer, the space above the fold varies according to screen size and configuration.
Above the fold issues are the same for mobile devices (tablets and smartphones) and emails.
The expression "above the fold" comes from the press because for newspapers the part seen by the potential buyer is above the fold when the newspaper is on display shelves.
See also scroll tracking.
Google Analytics deals with the issue by offering a function that allows web designers to see the proportion of visitors who can see a specific content.