Also known as "Confirmed Opt-in". The recipient has verifiably confirmed permission for the address to be included on the specific mailing list, by confirming (responding to) the list subscription request verification.
This is sometimes incorrectly referred to as "Double Opt-in". Example scenario: A visitor to your website types their email address in an opt-in form. The visitor is sent an email with a unique unguessable token, often in the form of a website link. The visitor must click on the unique link in that email message to verify their subscription request before being sent any marketing material. This verifies the owner of the email address attempting to be subscribed is indeed the person that typed it into the website form. Reference: http://www.spamhaus.org/mailinglists.html