What does it mean peripheral device?

A peripheral device is an internal or external device that connects directly to a computer or other digital device
but does not contribute to the computer's primary function, such as computing.
It helps end users access and use the functionalities of a computer.

Peripheral devices are those devices that connect to the core computing unit.
Four examples of peripheral devices are monitors, mice, keyboards, and printers.

Note: the pargraphs above are copied from Google.