ACS Access Control Server is application which enables issuing institution to participate in 3D secure program. It is either implemented in issuing bank premises or used by issuing bank from approved ACS SaaS platform provider.
ACS ensures that cardholder is authenticated during online card not present payment either through Risk based authentication or using other available authentication methods such as One time Password, SMS OTP, mobile token, biometry etc.
For more information download ACS datasheet.