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Orientation usually refers to the typical height and width of the screen when viewing a web page. The traditional variants are portrait, where the screen is usually taller than wide, or landscape, where the screen is wider than it is tall. If the browser window size is adjustable, the orientation of the screen may be irrelevant and the aspect ratio (ratio if width to height) is what's important.
Most desktop and laptop screens have fixed landscape-style orientation and most mobile devices will dynamically switch the orientation between a default of portrait and landscape when turned on its side (although some mobile devices are traditionally used in landscape mode most of the time).
Portability should support nearly any reasonable aspect ratio or screen orientation.