So what qualifies as 'High Definition' Video?

One basic criteria often referenced for high definition video is having at least four times the resolution of standard television (double the pixels horizontally and vertically). However...

A significant source of confusion for production professionals migrating into HD delivery is that the accepted picture sizes for digital television distribution vary and not all are “high definition” by any modern standard we could apply. In the United States, the ATSC or Advanced Television Systems Committee has issued “Table 3” of picture formats for digital television, specifying 640x480, 704x480, 1280x720, and 1920x1080 as digital video display resolutions. Both the 640x480 and the 704x480 formats have interlaced and progressive variations at multiple frame rates, with the progressive scan versions often referred to as “extended” or “enhanced definition,” the spoken shorthand being “EDTV.” (HighDef.org has the chart, as well as a simplified version here.)

The two accepted frame sizes for HD, 1920x1080 interlace/progressive, and 1280x720 progressive are 16:9 aspect ratio picture formats.

1280x720 is not 4x as many pixels as standard definition video (it even falls short of 3x), but it is used as a progressively scanned image. Most professionals would agree that progressive scanning does allow for the perception of very high detail with the tradeoff being progressive scanning does sacrifice some aspects of the smooth motion of interlaced scanning at an identical frame rate.

Advocates for the 1280x720 progressive format point out that while 1920x1080 images may be larger in size, when the 1080 frame is interlaced the picture delivers only half of the picture content every 60^th of a second, while “720p” can refresh the entire image every 60^th of a second. In this scenario then, 720p would deliver only slightly fewer pixels over time than “1080i,” and it would be temporally “sharper.”

The comparison and debate over spatial resolution (frame size) and temporal resolution (frame rate) and whether one trumps the other didn’t begin here and it won’t end here either. However, it's probably significant to note that one large stock footage library has ceased placing new footage acquired as anything less that 1080...and the trend for program format for an increasing number of broadcast and cable networks is to simply require a 1080i master with the stipulation that footage may be acquired as 720p (not mastered) with prior clearance... Many local stations will undoubtedly drag their feet on upgrading local origination to HD resolutions in the present economy, but mastering you content in HD maximizes shelflife.

Each professional needs to take a hard look at what sort of projects they do and what kind of delivery requirements are involved. It does look as if mainstream program distribution is definitely pointing toward 1080.