motherboard is the primary circuit board in a computer. It typically has sockets for the machine’s CPURAM, keyboard, mouse, and disk controllers, and the circuitry to interconnect them all. It’s also common for motherboards to contain additional circuitry for external connections, such as USB and eSATA. Many motherboards also include video adapter circuitry as well.

Many motherboards, particularly those in desktop PCs, include slots into which expansion cards can be inserted to provide additional hardware functionality. Occasionally, those expansion cards, or other cards that can be attached to a motherboard, may be referred to as daughterboards.

motherboard (Wikipedia)

A motherboard (also called mainboard, main circuit board, or mobo) is the main printed circuit board (PCB) in general-purpose computers and other expandable systems. It holds and allows communication between many of the crucial electronic components of a system, such as the central processing unit (CPU) and memory, and provides connectors for other peripherals. Unlike a backplane, a motherboard usually contains significant sub-systems, such as the central processor, the chipset's input/output and memory controllers, interface connectors, and other components integrated for general use.

Motherboard means specifically a PCB with expansion capabilities. As the name suggests, this board is often referred to as the "mother" of all components attached to it, which often include peripherals, interface cards, and daughterboards: sound cards, video cards, network cards, host bus adapters, TV tuner cards, IEEE 1394 cards; and a variety of other custom components.

Dell Precision T3600 System Motherboard, used in professional CAD Workstations. Manufactured in 2012

Similarly, the term mainboard describes a device with a single board and no additional expansions or capability, such as controlling boards in laser printers, television sets, washing machines, mobile phones, and other embedded systems with limited expansion abilities.

Motherboard for a desktop personal computer, showing the typical components and interfaces that are found on a motherboard. This model follows the Baby AT (form factor), used in many desktop PCs.
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