Scanners, printers, and speakers are peripheral devices for a computer because they aren't central to the working of the computer itself. Anything peripheral is on the margin, or outside, while main things, like a computer's processor, are not peripheral.

Outside of computer jargon, peripheral is a common term in eye exams, which measure peripheral vision to see how far toward the outer right and left edges you can see. Peripheral also describes nerves from the body's peripheral system, which are secondary to the main system but branch off of it — like the wires between peripheral devices and a computer.

Primary Meanings of peripheral

on or near an edge or constituting an outer boundary; the outer area
(computer science) electronic equipment connected by cable to the CPU of a computer
Full Definitions of peripheral

adj on or near an edge or constituting an outer boundary; the outer area

“Russia's peripheral provinces”
peripheral suburbs”
lying around or just outside the edges or outskirts
fringy, marginal
at or constituting a border or edge
encircling, skirting
being all around the edges; enclosing
located outside a military base
in or near a center or constituting a center; the inner area
located in the middle part of a ship or aircraft
having two centers
having two foci
center, halfway, middle, midway
equally distant from the extremes
being or placed in the center
centric, centrical
having or situated at or near a center
having or localized centrally at a focus
medial, median
relating to or situated in or extending toward the middle
middlemost, midmost
being in the exact middle
constituting or like a nucleus
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adj related to the key issue but not of central importance

“a peripheral interest”
“energy is far from a peripheral issue in the economy”
peripheral issues”
incident, incidental
(sometimes followed by `to') minor or casual or subordinate in significance or nature or occurring as a chance concomitant or consequence

n (computer science) electronic equipment connected by cable to the CPU of a computer

“disk drives and printers are important peripherals
computer peripheral, peripheral device
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data input device, input device
a device that can be used to insert data into a computer or other computational device
output device
electronic or electromechanical equipment connected to a computer and used to transfer data out of the computer in the form of text, images, sounds, or other media
(computer science) an output device that prints the results of data processing
computer keyboard, keypad
a keyboard that is a data input device for computers; arrangement of keys is modelled after the typewriter keyboard
dot matrix printer, dot printer, matrix printer
a printer that represents each character as a pattern of dots from a dot matrix
a manual control consisting of a vertical handle that can move freely in two directions; used as an input device to computers or to devices controlled by computers
digital scanner, image scanner, scanner
an electronic device that generates a digital representation of an image for data input to a computer
Type of:
electronic equipment
equipment that involves the controlled conduction of electrons (especially in a gas or vacuum or semiconductor)