The term 720p refers to 720 lines of progressive video (720 lines per frame). Higher resolution than standard DVD, which is 480i or 480p. Usually refers to 1280x720 resolution in 1.78 aspect ratio. [Source: LG Guide to HDTV]
See also HDTV.
Also see Webopedia's "Did You Know...All About Digital TV . DTV and HDTV".