From The Free On-line Dictionary of Computing (19 January 2023):
motherboard
mobo
system board
<hardware> (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)