From The TV IV
1080i is the name given to a specific HDTV video mode. The number 1080 stands for 1080 lines of vertical resolution, while the letter i stands for interlaced or non-progressive scan. It is currently the highest quality interlaced video mode available for HDTV, although people argue that since many HDTVs are Plasma, LCD, or DLP based, that there is no point in having the source interlaced. This is done simply because most forms of 1080i require less bandwidth than 1080p. 1080i is the format used by CBS, NBC, UPN, and The WB for their standard HDTV broadcasts.
1080i resolution is 1920 × 1080.