1080i is the shorthand name for a category of video modes. The number 1080 stands for 1080 lines of vertical resolution, while the letter i stands for interlaced or non-progressive scan. 1080i is considered to be an HDTV video mode. The term usually assumes a widescreen aspect ratio of 16:9, implying a horizontal resolution of 1920 pixels and a frame resolution of 1920 1080 or about 2.07 million pixels, and a field resolution of 1920 1080 / 2 (because it's interlaced) or about 1.04 million pixels. The field rate (not the frame rate) in hertz can be either implied by the context or specified after the letter i. The two field rates in common use are 50 and 60 Hz, with the former (1080i50) generally being used in traditional PAL and SECAM countries (Europe, Australia, much of Asia, Africa), the latter (1080i60) in traditional NTSC countries (e.g. United States, Canada and Japan). Both variants can be transported by both major digital television formats, ATSC and DVB.
http://en.wikipedia.org/wiki/1080i