Source : Free On-Line Dictionary of Computing

power-on self-test
     
         (POST) A sequence of diagnostic tests that are
        run automatically by a device when the power is turned on.
     
        In a {personal computer} a typical POST sequence does the
        following:
     
        - checks that the {system board} is working
     
        - checks that the {memory} is working
     
        - compares the current system configuration with that
        recorded by the PC's configuration program to see if
        anything has been added or removed or broken
     
        - starts the video operation
     
        - checks that the {diskette} drive, {hard disk drive},
        {CD-ROM} drive, and any other drives that may be installed
        are working.
     
        When POST is finished, typically it will {beep}, and then
        let your {operating system} start to {boot}.  If POST finds
        an error, it may beep more than once (or possibly not at all
        if it is your PC speaker that is broken) and display a POST
        error message.  These messages are often nothing more than a
        single ominous number.  Some common numbers and their
        meanings are:
     
        161 Dead battery
        (get a new battery for the system board)
     
        162 Configuration changed
        (you added some memory or a new card to the PC)
     
        301 Keyboard error
        (take the book off the corner of the keyboard)
     
        Because a successful POST indicates that the system is
        restored to known state, turning the power off and on is a
        standard way to reset a system whose software has {hung}.
        Compare {120 reset}, {Big Red Switch}, {power cycle}.
     
        (2001-03-30)