An opt-in email list contains the email addresses of users who have explicitly indicated their willingness to receive emails from a commercial organization.
A distinction is commonly made between single opt-in and double opt-in files. In a single opt-in, the user gives their permission once, while in a double opt-in, this permission is confirmed twice by the user. An adaptation of this is the 'confirmed opt-in', where, after subscribing, the user receives an additional confirmation email to verify their registration.