Confirmed opt-in as defined by SpamHaus, who is one of the most respected anti spam organizations in the world:

Known as “COI” in the legitimate bulk email industry, also known as “Confirmed Opt-in”, “Verified Opt-in” or sometimes “Double Opt-in”.

With Closed-Loop 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 the standard practice for all responsible Internet mailing lists, it ensures users are properly subscribed, from a working address, and with the address owner’s consent.

In the event of “spam” accusation:

The Bulk Email Sender is fully and legally protected because the reply to the Subscription Confirmation Request received back from the recipient proves that the recipient did in fact opt-in and grant verifiable consent for the mailings.
Source:Spamhaus Website

Numerous myths have circulated regarding confirmed opt-in and its effects. There are many misconceptions out there, and we’d like to help clear those up.