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An 'authentication factor' is a piece of information used to authenticate or verify a person's identity for security purposes.
The three most commonly recognized factors are:

★ 'Something you know', such as a password or PIN

★ 'Something you have', such as a credit card, hardware token, or telephone.

★ 'Something you are', such as a fingerprint, a retinal pattern, or other biometric.
Other, less common factors may include:

★ Recognition-based or ''cognometric'' authentication, such as Passfaces^TM, where the user has to recognize pre-assigned secret faces

★ Cybermetric authentication, such as only allowing access from the certain computer, which is the combination of unique hardware and (or) software installed

★ Location-based authentication, such as only allowing a particular atm, charge, or credit card to be used at a specific merchant or at a specific bank branch, or only allowing root access from specific terminals

★ Time-based authentication, such as only allowing access from certain accounts during normal working hours

★ Size-based authorization, such as only allowing a specific transaction to be for a specific exact amount

★ Pre-authorized transactions, such as where a company uploads all of the check numbers and amounts written for each check to their bank, and the bank would then reject any check not of those numbers and amounts as fraudulent