A personal identification number sent to its user in a letter. The darkened paper flap prevents the number from being read by holding the unopened envelope to the light. A personal identification number (PIN, pronounced "pin"; is often spoken out loud "PIN number" by mistake) is a numeric or alpha-numeric password or code used in the process of authenticating or identifying a user to a system and system to a user.
PIN is commonly used in mobile phones where after three tries of entering the pin code the user is asked to inter the Personal unblocking code.
The personal identification number has been the key to flourishing the exchange of private data 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.