The Free On-line Dictionary of Computing (30 December 2018):

power-on self-test
POST

    (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)