The Collaborative International Dictionary of English v.0.48:

Peripheral \Pe*riph"er*al\, a.
   1. Of or pertaining to a periphery; constituting a periphery;
      peripheric.
      [1913 Webster]

   2. (Anat.) External; away from the center; on or near the
      surface; as, the peripheral portion of the nervous system.
      [1913 Webster]

   3. Pertaining to minor, unimportant, superficial, or
      irrelevant aspects of the matter in question; as,
      peripheral isssues are dealt with in the appendix.
      [PJC] Peripheric

WordNet (r) 3.0 (2006):

peripheral
    adj 1: on or near an edge or constituting an outer boundary; the
           outer area; "Russia's peripheral provinces"; "peripheral
           suburbs" [ant: central]
    2: 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"
    n 1: (computer science) electronic equipment connected by cable
         to the CPU of a computer; "disk drives and printers are
         important peripherals" [syn: peripheral, computer
         peripheral, peripheral device]

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "peripheral":
   ambient, apparent, beside the point, circling, circumambient,
   circumferential, circumflex, circumfluent, circumfluous,
   circumjacent, cortical, delineatory, embracing, encircling,
   enclosing, encompassing, enfolding, enveloping, environing,
   epidermic, exomorphic, exterior, external, extrinsic, fringe,
   in outline, incidental, inessential, irrelevant, minor,
   neighboring, open, out, outer, outermost, outlined, outlying,
   outmost, outside, outstanding, outward, outward-facing, perimetric,
   public, roundabout, secondary, seeming, suburban, superficial,
   surface, surrounding, tangential, unessential, unimportant,
   unnecessary, wrapping



The Free On-line Dictionary of Computing (30 December 2018):

peripheral
device
peripheral device

    (Or "peripheral device", "device") Any part of a
   computer other than the CPU or working memory,
   i.e. disks, keyboards, monitors, mice, printers,
   scanners, tape drives, microphones, speakers,
   cameras, to list just the less exotic ones.

   High speed working memory, such as RAM, ROM or, in the
   old days, core would not normally be referred to as
   peripherals.  The more modern term "device" is also more
   general in that it is used for things such as a pseudo-tty,
   a RAM drive, or a network adaptor.

   Some argue that, since the advent of the personal computer,
   the motherboard, hard disk, keyboard, mouse, and monitor
   are all parts of the base system, and only use the term
   "peripheral" for optional additional components.

   (2002-09-03)