The 16:9 aspect ratio (also known as widescreen) is an aspect ratio that is 16/9 or 1.78 times as wide as it is high, i.e. 16 units wide per 9 units tall.
16:9 is not the only widescreen format. Anamorphic widescreen (2.35:1) and American theatrical standard (1.85:1) have wider aspect ratios, while the European theatrical standard (1.66:1) is just slightly less. (Imax, contrary to some popular perception, is 1.33:1, the traditional television aspect ratio.)
16:9 supporters state that the wider picture corresponds much better to the human visual field than the almost square 4:3.