peripheral (adj.)

1803, "of, belonging to, or situated on the periphery," from periphery + -al (1). Earlier formations were peripherial (1670s); peripherical (1690s). Related: Peripherally. As a noun, peripherals, "peripheral devices of a computer," is by 1966.

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Definitions of peripheral from WordNet
peripheral (adj.)
on or near an edge or constituting an outer boundary; the outer area;
peripheral suburbs
Russia's peripheral provinces
peripheral (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
peripheral (n.)
electronic equipment external to the circuit board that contains the CPU of a computer;
disk drives and printers are important peripherals
Synonyms: computer peripheral / peripheral device