Transaction authentication number, TAN or T.A.N. is used by some online banking services as a form of single use one-time passwords to authorize financial transactions. TANs are a second layer of security above and beyond the traditional single-password authentication.
TANs are believed to provide additional security because they act as a form of two-factor authentication. Should the physical document or token containing the TANs be stolen, it will be of little use without the password; conversely, if the login data are obtained, no transactions can be performed without a valid TAN.