Authenticity is the quality of being real, genuine, or true — not fake, not copied, not put on. You can talk about the authenticity of a painting, a signature, or an old letter: is it the real thing? People also use it for food, music, and places (“the authenticity of this little noodle shop”). In the last few decades it has stretched further: “be authentic” can mean “be true to yourself, don’t wear a false face.” That later use is common in magazines and self-help talk; it is a bit looser than the older “not a fake object” meaning, though actually both sit on the same idea of not pretending. It is a fairly formal noun; the everyday adjective is authentic.