System Unit

When you need to refer to the box that houses your computer, use this term. The system unit refers to the computer itself, which does not include the monitor, the keyboard, the mouse, or any other peripherals. It includes the chassis, motherboard, processor, memory, video card, hard drive, and ports. I suppose most people will probably know what you mean when you say, "the box", but saying "system unit" will definitely make you look more sophisticated.