The actors were dressed in 'authentic' medieval costumes. [=costumes that looked just like real medieval clothes]
The witness provided an 'authentic' record/report/account of what actually took place.
Experts have confirmed that the signature on the letter is 'authentic'.
(..) So, what time does the costume parade start? The parade? Yeah, so the judges can give out the prizes for best costume. You know, most frightening, most authentic, most accurate visualization of a scientific principal. Oh, Sheldon, I'm sorry, but there aren't going to be any parades (..)2007 TvShow: The Big Bang Theory Title: The Middle Earth Paradigm Season: 1 Episode 6