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.

Definitions of peripheral
  1. adjective
    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
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    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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  2. adjective
    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
  3. noun
    (computer science) electronic equipment connected by cable to the CPU of a computer
    “disk drives and printers are important peripherals
    synonyms: 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)
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