The Collaborative International Dictionary of English v.0.48:

motherboard \moth"er*board`\ n. (Electronics)
   The board containing the main circuits of an electronic
   device, especially computers. The term is used primarily in
   microcomputer literature, where it designates the board
   containing the main expansion bus, and usually also the cpu.
   On motherboards designed with an expansion bus, often all of
   the circuits not contained on an expansion card are on the
   motherboard.
   [PJC]

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)