Peripheral
Peripherals are devices attached to a computer in order to expand its functionality. When we see a computer, what we can directly see is a set of peripherals. In other words motherboard, CPU and memory are not peripherals. So, peripherals are generally visible to everybody. In the computer world, peripherals include monitor, mouse, modems, floppy disk drive,USB flash drive, keyboard, printers, scanners, CD drivers, speakers, etc.
There is some argument over whether devices such as monitors, disk drives, etc should considered peripherals when they are not truly optional, and typically internal devices are not referred as peripherals.
When computer technology evolves you may get more and more peripherals. Basically the input, output and auxiliary storage devices attached to a computer are known as peripherals.