All High Definition (HD) means is that it is "720p" or more. This just means there are at least 720 horizontal lines of resolution. Anything with a resolution of 1280x786 is automatically considered HD, because it has 786 horizontal lines of resolution.
A lot of products, like computer monitors, have been made for many years at resolutions higher than 720p, but the term HD wasn't used until they started selling TV's at above normal broadcast resolution (TV signals used to all be 480p). Now they sell a lot of stuff with the HD label, even though it has been HD all along by the definition.
Summary: Ignore the "HD" and just look at the resolution.