When you authenticate something, you establish that it’s the real thing. If you authenticate a painting, for example, you’re sure that it's an original work, not a copy.
The verb authenticate came into English in the seventeenth century by way of the Latin word authenticat-, meaning “established as valid.” You’ve probably heard it to mean that a valuable item, like a diamond or a rare vase, is genuine. But not everything that has to be authenticated is worth a lot of money. For example, experts authenticate evidence in an investigation and you authenticate your identity to gain access to your email account.