One might say that HDTV is 720P and 1080i , and then Full HDTV is 1080P. <><> And then one might get in to , i = interlaced scanning , and P = progressive scanning. And then one might know that i- interlaced scan lines do not quite match up and what this does is , have you ever noticed that stripes in the picture striped ties and striped shirts have a rainbow effect , and if the tv camera is moved up and down or down and up , the picture has a wavy effect , that's i-interlaced scanning. <><> Progressive scanning will have little or none of this. <><> So any way , one might call it a marketing thing. <><> Humans like the WOW! factor. Like --> WOW!!!!! it's FULL HDTV , the eyes light up , the nostrils flare open , the chest puffs out , and then the person relaxes with a satisfied feeling. <><> So here is the truth as I see it , The Digital Pictures of 480i and 480P <--low definition and 720P , 1080i , 1080P <--high definition are nice clear pictures , no ghost , no bad colors , no drifting lines , no snow that analog tv has. <><> Digital TV has a nice clear picture picture and I like that.
The original answer is correct in that it is mostly marketing but it has come to symbolize the difference between 720p/1080i vs. 1080p. Many Blu-ray discs, PS3 and Xbox360 games as well as Televisions will boast Full HD. It simply means it has the capability of outputting 1080p.