Source : Webster's Revised Unabridged Dictionary (1913)

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

   2. (Anat.) External; away from the center; as, the peripheral
      portion of the nervous system.

Source : WordNet®

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"

peripheral
     n : (computer science) electronic equipment connected by cable
         to the CPU of a computer; "disk drives and printers are
         important peripherals" [syn: {computer peripheral}, {peripheral
         device}]

Source : Free On-Line Dictionary of Computing

peripheral
     
         (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)