In marketing, opt-in is a term used for defining those forms or e-mails that are shown or sent to users in order to obtain their permission to contact them with further messages. The opt-in is the consent given by users, which acknowledges their interest in your products or services and authorizes you or third parties to send them further promotional or informative messages.

Opt-in forms can be displayed as pop-ups, landing pages, widgets, or can be sent via e-mails, and are required by law, just like opt-out forms.