A Personal Identification Number (PIN) is a numerical code used for user authentication. Typically consisting of 4 to 6 digits, PINs are commonly used with debit and credit cards, mobile devices, and some online services. PINs are considered a form of knowledge-based authentication, as they rely on something the user knows. However, the relatively short length and numeric nature of PINs make them less secure than complex passwords. As a result, they are often used in conjunction with other authentication methods, such as biometrics or tokens.