According to Crutchfield Advisor
, it doesn't mean anything. It's like the S in Harry S. Truman. iBiquity trademarked the name "HD Radio," and it don't stand for shit except money.
They sure aren't stopping people from assigning meaning to it, though. "High Definition" and "Hybrid Digital" are techy sounding terms that do the legwork of making people think
HD Radio is hot shit.
Indistinguishable from magic, and all that.