Why is external hardware also known as peripheral?

A Peripheral is a input or output hardware device that could connect to a computer internally or externally, usually we know about external peripherals like monitors, mice, keyboards and printers. These are not required to run computer but they give the computer additional capabilities. For Example, a computer monitor isn't technically required in computing, but it's actually necessary to use the computer. When we buy a new desktop computer or a laptop, some of the peripheral come with it as an internal peripheral like in a laptop there is CD-ROM drive, Hard drive or Video card.

So most peripherals are external hardware devices. However, a peripheral could be an external or internal device.