One of the resolution specs used in the HDTV. 720i stands for resolution of 1280x720 pixels and the magic little 'i' means that the video is in interlaced format. Other common HDTV resolutions are 1080i and 720p.
The term in itself however, is erroneous and is usually a typo. The user of the term is almost certainly talking about 720p as there is no broadcast standard that allows for 720 interlaced lines in a video at any frame rate, including PAL or NTSC.