pe·riph·er·y / pəˈrifərē/ • n. (pl. -er·ies) the outer limits or edge of an area or object: new buildings on the periphery of the hospital site. ∎  a marginal or secondary position in, or part or aspect of, a group, subject, or sphere of activity: a shift in power from the center to the periphery.

periphery †layer of air XIV (once); boundary of a rounded surface XVI. — late L. peripherīa — Gr. periph´reia, f. peripherḗs revolving round, f. PERI- + phérein BEAR2; see -Y3.
Hence peripheral XIX.