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.
The main circuitboard inside a computer. It works with the operating system to control all the resources of the computer. Any function that is provided on the motherboard, versus on a separate, add-on circuitboard, is called "on-board".
Everything attached to your computer connects to the motherboard directly or indirectly. It generally houses the CPU chip, the controller circuitry, the bus, and sockets for additional boards, which are called daughterboards.