1. To verify or to prove one’s identity: Before Jane could check her e-mails, she first had to authenticate her e-mail address and password.

Before he could pick up his package at the post office, Jeffery had to authenticate that he was the authorized person by showing his driver’s license.

2. To prove something to be genuine; especially, some artistic piece of work: The insurance company wanted Linda to have her violin's condition authenticated by a violin expert so it could be insured in case of theft, fire, etc.