I don't have a strict definition handy, however it seems as though digital currencies would be those that are digitally based, whereas the dollar is a tangible currency first, even if it is fiat.

A digital currency would be like crypto currency, or in-game currencies or something designed to be digital. The dollar is the federal reserve currency, and not intended to be digital. It doesn't make sense to call it digital currency, when in fact it's physical currency first, and we've developed digital tools to help us use it and move it around.

At this point the vast majority of USD is digitally represented only.

I don't disagree, but it doesn't change that USD is not a digital currency.