I can't find any official definition, just something relating to Intel's standard. For example, HD video is >= 1280 lines x 720 lines. Is bit rate a factor? Lately I've been looking at my sound settings to see the different device options and ports/jacks. Until now the on board audio was disabled. After I enabled it temporarily I see 2 extra devices(speakers and digital output at the bottom).
Very top is my monitor, which doesn't have speakers. Speakers are a 2.1 system via Line out. You can see the "Realtek High Definition Audio" caption there. It doesn't say high def for the devices in the sound card, but it does for the on board ones. Seems like it would be the other way around.