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

Uninterruptible Power Supply

    (UPS) A battery powered power supply unit that is
   guaranteed to provide power to a computer in the event of
   interruptions in the incoming mains electrical power.
   Different rating UPSs will provide power for different lengths
   of time.

   Modern UPSs connect to the computer's serial port and
   provide information such as battery time remaining, allowing
   the computer to shut down gracefully before complete loss of
   power.

   (1996-12-11)