What is the X in X-mas?
late 14c., "atmosphere around the earth," from Old French periferie (Modern French périphérie), from Medieval Latin periferia, from Late Latin peripheria, from Greek peripheria "circumference, outer surface, line round a circular body," literally "a carrying around," from peripheres "rounded, moving round, revolving," peripherein "carry or move round," from peri- "round about" (see peri-) + pherein "to carry" (see infer). Meaning "outside boundary of a surface" attested in English from 1570s; general sense of "boundary" is from 1660s.
periphery pe·riph·er·y (pə-rĭf'ə-rē)
The outermost part or region within a precise boundary; the part away from center.
The outer surface of a solid.