It seems arguing from authority v. arguing based on the opposing side's ignorance are similar, but different.

The difference is no authority is mentioned in the ad ignorantium argument. By virtue of the fact that the opponent can cite no evidence, you claim that your position is true. I win because you can't prove your point.

In searching the web to learn more about ad ignorantium arguments, I realized that much of what is thrown at us as being true is based on the fact that the opposition didn't have evidence enough to support their thesis. Well, I must be right if you can't provide a shread of evidence to show that the your point is valid.

What is the latinate term for proposing arguments based on authority? I do remember coming across that term as another example of fallacious argument.

After you study these fallacies for a short while, you do begin to realize that there's a mountain of fallacious arguments out there--just as Faldage wrote about above.