480i refers to interlaced video encoded with 240 line fields, forming a final Frame 480 Pixels tall. Although 480i is sometimes used to refer to NTSC, they're not the same thing. It can be used to refer to a NTSC signal that's been digitized, such as captured analog TV, assuming the Capture conforms to the 480 line standard established by ITU-R BT.601.
In addition to captured NTSC, other common formats using 480i video are Standard Definition ATSC, EDTV, DVD, DV, and SVCD. 480i is most suitable for TV content shot or edited on videotape or other analog tape sources like home videos.
Regardless of the format it's encoded in, or exact resolution of a 480i video file, they generally have the standard NTSC framerate of 29.97fps, although in rare cases it may be 30fps instead.