3D Secure is a collective term for the security standards Verified by Visa
and MasterCard SecureCode
which are developed by Visa and MasterCard, respectively.
When 3D Secure is used, an extra security measure is added to the payment process. After the customer has entered her credit card details, she will receive a one-time password on sms, which she has to enter to complete the purchase. You can see how the password might look in image 1 below.
3D Secure only applies to international payment cards and cannot be used with Dankort payments.
Please note that some acquirers require the use of 3D Secure. Contact the acquirer for further information.