It sounds like you are asking two different questions.
See the links below for
"What is the definition of system unit?" and
"What is the definition of a motherboard?".
Motherboard
A computer system unit is the case with the motherboard and all the ports. The box you unplug and walk around with. The CPU is the Central Processing Unit, this is a chip on the motherboard that does all the processing.People erroneously refer to system unit as CPU.
Motherboard
Motherboard or system board
There is a wide difference between the internal system unit and the external system unit. The internal system unit is usually mounted on the motherboard inside the computer while the external system unit is independent of the computer like an external drive.
The system unit is the box where all processing takes place. All other devices are attached to it. The Central Processing Unit (CPU) is the heart of the computer, and is inside the system unit.
system board i.e also known as motherboard.
A system unit, also known as a S.U., is the main unit of a personal computer, typically consisting of a metal or plastic enclosure containing the motherboard, power supply, cooling fans, internal disk drives, and the memory modules and expansion cards that are plugged into the motherboard.
The hardware unit that contains the CPU, the RAM, a hard disk, and a power supply is called the "motherboard."
The main circuit board of a computer's system unit is called the motherboard. Motherboards feature different form factors e.g. ATX, BTX and NLX.
The main circuit board of a computer's system unit is called the motherboard. Motherboards feature different form factors e.g. ATX, BTX and NLX.
Motherboard, ram, hardisk, power supply, video card