: not relating to the main or most important part
computers : connected to a computer but not an essential part of it
: of or relating to the area that is to the side of the area you are looking at
computers : a piece of equipment (such as a printer, speaker, etc.) that is connected to a computer but is not an essential part of it
What made you want to look up peripheral? Please tell us where you read or heard it (including the quote, if possible).