of undisputed origin; genuine.
“the letter is now accepted as an authentic document”
genuine, real, bona fide, true, veritable;
legitimate, lawful, legal, valid;
informal – the real McCoy, the real thing, kosher
Above is Google’s cookie-cutter definition of authenticity. It has taken me over twenty-four years to even begin letting this word resonate in my 21st century-paced mind.