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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)