The term 3D Secure comes from Three Domain Security. This is due to the fact that there are 3 "domains" involved in the 3D Secure process: Issuer Domain, Interoperability Domain and Acquirer Domain.
- The issuer domain is where the cardholder and issuing bank act.
- The acquirer domain is where the gateway and acquiring bank act.
- The interoperability domain is where all the "connecting" services act (Directory Server & ACS).
See our article here on how 3D Secure works.