Viewport
The viewport is the portion of a Web browser 's interface within which the content of a Web site is displayed.
When a Web page is longer or wider than the size of the viewport, the visitor will have to scroll to view the full page. Especially on devices with smaller screens, such as smartphones, the ability to zoom in and out to show the full page (or part of it) is widely used.
The part of a Web page that is immediately visible when the page is opened is also called above the fold, by analogy with the front page of a folded newspaper. The part that requires scrolling is then located below the fold.
Within responsive Web design, the layout of a Web page is tailored, among other things, to the height and width of the viewport. In the page's source code, a meta tag called viewport can be used to set the size at which the page should be displayed when opened and the extent to which the visitor is allowed to zoom in and out.
For banner positions within a web page, their visibility is sometimes expressed as viewability. This is expressed as a percentage of the number of times a banner was actually seen by the visitor compared to the total number of placements.
