A numerical code used in many electronic financial transactions. Personal identification numbers (PINs) are usually used in conjunction with usernames or other passwords. They are also usually required when using bank debit or credit cards, and most banks or financial institutions issue PINs separately from the cards through the mail. |||PINs usually contain four digits. Not all electronic transactions use this number, but cash withdrawals from ATMs invariably require this code. PINs should be known only to the users and never disclosed to anyone else.