Email that is explicitly requested by the recipient.
The definition of opt-in email has been a matter of intense debate.
“single opt-in” vs. “double opt-in”
The term single opt-in simply means that actions were taken to sign up for the email in question. The term double opt-in means that the subscriber has actively confirmed their subscription, typically by responding to an automatically-generated message sent to the email address. Proponents of double opt-in may not actually use that term, as they feel any email labled “opt-in” must be verified.
see also: Email Marketing Tools