A specific, pro-active, request by an individual email recipient to have their own email address placed on a specific mailing list. Many list renters and buyers now require list owners to provide proof of opt-in, including the actual email or IP address date and time the request was received.

The 3 Things To Avoid When Emailing Your List
When you decide to have an opt-in list, it is not just a matter of sending your subscribers your promotional newsletters or catalogs.