Double opt-in is a two-step consent process in which a user first submits their consent — by checking a box or clicking "accept" — and then confirms it through a second action, typically clicking a verification link sent to their email. Double opt-in is most commonly used in email marketing and newsletter sign-ups to confirm the validity of an address and ensure the user genuinely intended to opt in. While GDPR does not mandate double opt-in, it is best practice for demonstrating documented, unambiguous consent — especially for marketing communications where consent is the lawful basis for processing.