Double Opt-In (Definition)
Often time the wording used by spammers to imply the recipient has "opted-in twice".
The first time, says the spammer, was when the address was obtained and "opted-in" by the spammer without the recipient's consent. The second time was when the recipient failed to opt-out after receiving spam.
The correct method for securing a mailing list is by the usage of Confirmed Opt-In also called Verified Opt-In.