What is Personal Identification Number?

Legal Definition
A personal identification number (PIN, pronounced "pin"; often redundantly PIN number) is a numeric password used to authenticate a user to a system.

PINs may be used with banking systems to authenticate the identity of the cardholder, but are also used in other, non-financial systems.

Usually, both a private PIN and public user identifier is required to authenticate a user to the system. In these situations, typically the user is required to provide a non-confidential user identifier or token (the user ID) and a confidential PIN to gain access to the system. Upon receiving the user ID and PIN, the system looks up the PIN based upon the user ID and compares the looked-up PIN with the received PIN. The user is granted access only when the number entered matches with the number stored in the system. Hence, despite the name, a PIN does not personally identify the user. The PIN is not printed or embedded on the card but is manually entered by the cardholder during automated teller machine (ATM) and point of sale (POS) transactions (such as those that comply with EMV), and in card not present transactions, such as over the Internet or for phone banking.

PINs may also be used in contexts other than ATM or POS transactions, such as door access, Internet transactions, or to log in to a restricted website. In such cases, the PIN may simply be a password, and not necessarily associated with a specific card.
-- Wikipedia
Legal Definition
A secret combination of numbers, or letters which can be used to make use of a credit card, ATM, gain access to a computer, etc.