The system unit refers to the main computer case and the internal components housed inside it. In Information Technology hardware terminology, the system unit contains the core electronic parts required for processing and storage, including the motherboard, CPU, RAM, storage drives, power supply, and expansion cards. The motherboard is a central internal component of the system unit because it is the main circuit board that holds and connects major hardware parts through sockets, slots, and buses. By contrast, a keyboard is an external input peripheral, a monitor is an external output display device, and a printer is an external output peripheral used for hard-copy printing. While these peripherals are essential for user interaction, they are not considered part of the system unit itself. Understanding this distinction is important for tasks such as hardware troubleshooting, upgrades, and system assembly, where internal components are serviced within the system unit enclosure. Therefore, the component that is part of the system unit is the motherboard.