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

motherboard
mobo
system board

    (mobo) The main printed circuit board in an
   electronic device, particularly a computer, which may contain
   sockets that accept additional boards ("daughter-boards").

   In a personal computer, the motherboard contains the bus,
   the microprocessor, and integrated circuits used for
   controlling any built-in peripherals such as the keyboard,
   text and graphics display, serial ports and parallel
   ports, joystick, and mouse interfaces.

   (2000-08-10)