Actually, that is not correct. This is a pure marketing term invented by iBiquity to promote their digital radio in a simple to understand way.I know as I worked directly with them. It is marketing speak and stands for nothing specific.
Thanks Rheiblim :) My understanding is that it started out as Hybrid Digital, then became purely a marketing term (like DVD). But if you worked with them, then you're a more reliable source than, say, Wikipedia ;)