Two Factor Authentication [2FA] is what happens when you try to log in somewhere and it requires a second, additional authorization to complete logging in. (Not just some kind of a login username & password.) Such additional authentication may be in the form of an email sent to you at the email address on file there, or a text message sent to the phone number on file, or an actual phone call to that number, or some other method. This is to prevent unauthorized persons logging in, who somehow obtained or guessed your username and password. Compares to Multi-Factor Authentication.
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