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'.
(..) but we have everything. Just a minute. You mean to tell me he's a robot? That's what l mean to tell you. We're paying $ a day for this? lt's authentic. The West of the s. Well, at least they could've made it a little more comfortable. But that's the point. This is really the way it was. (..)1973 Westworld