The term Opt-in (as it pertains to email marketing), or Email Opt-In, refers to the action where an individual chooses to become part of an email list.
Example of an Opt-in: Jane is on a gardening blog and chooses to opt-in to their newsletter.
How Opt-In Works
In most cases, a user opts-in to an email marketing list by submitting a Web form containing their email address. By doing so, the user forms a relationship with the email list owner, who now has permission to send marketing emails to the user.