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
The Free On-line Dictionary of Computing (26 July 2010):
(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.