A personal identification number (PIN), or sometimes redundantly a PIN number or PIN code, is a numeric (sometimes alpha-numeric) passcode used in the process of authenticating a user accessing a system.

The PIN has been the key to facilitating the private data exchange between different data-processing centers in computer networks for financial institutions, governments, and enterprises. PINs may be used to authenticate banking systems with cardholders, governments with citizens, enterprises with employees, and computers with users, among other uses.

In common usage, PINs are used in ATM or POS transactions, secure access control (e.g. computer access, door access, car access), internet transactions, or to log into a restricted website.