Is a process to authenticate users on which requires the presentation of two or more authentication factors. For example, a user introduces a password in a system, the system sends a code, or calls to the user cell phone, the user finally authenticates by introducing the code received.
Reduces drastically the incidence of online identity theft, phishing expeditions, and other online fraud, because the victim's password would no longer be enough to give access -- but may be vulnerable to other attacks.