Source : WordNet®

personal identification number
     n : a number you choose and use to gain access to various
         accounts [syn: {PIN}, {PIN number}]

Source : Free On-Line Dictionary of Computing

Personal Identification Number
     
         (PIN, "PIN number") A {password}, typically four
        digits entered through a telephone keypad or automatic teller
        machine.
     
        (1996-11-03)