Definition: The term "opt-in" refers to email marketing and email newsletters where the customer or reader has actively given permission (opted) to receive the emails in question. It is different from "Spam," which is unsolicited email.
The opt-in process benefits marketers because users are more likely to open and read email that they have asked to receive.
"Double opt-in," where the reader must also confirm his/her permission (usually by replying to a confirmation e-mail), is increasingly used by marketers.

Defining Opt-In Email Marketing
In the context of email marketing, it never takes long for catchwords like "opt-in" or "permission-based" to appear. But what exactly is opt-in email marketing?
I find it easiest to think in terms of mailing lists when it comes to email marketing. Email marketing is nothing else but mailing lists in the end. The mailing list of an opt-in marketing campaign starts empty. The only way for an email address to get onto the list is by its owner explicitly requesting it.
The request must always be for a specific list. If a customer submits a form that says "Yes, send me a copy of your current catalogue" you can add them to the list of customers who will receive a copy of the current catalogue, use the list one time and then throw it away. To continue sending them your latest catalogue, you need the explicit permission to do so (along the lines of "Yes, keep me up to date by subscribing me to your list of catalogue recipients").
"Permission-based" means essentially the same.
Marketing material is sent only with permission, and often personalized.
- Article by Wale Bello