The Collaborative International Dictionary of English v.0.48:

Periphery \Pe*riph"er*y\, n.; pl. Peripheries. [L. peripheria,
   Gr. perife`reia; peri` around + fe`rein to bear, carry: cf.
   F. p['e]riph['e]rie.]
   1. The outside or superficial portions of a body; the
      surface.
      [1913 Webster]

   2. (Geom.) The circumference of a circle, ellipse, or other
      figure.
      [1913 Webster]

WordNet (r) 3.0 (2006):

periphery
    n 1: the outside boundary or surface of something [syn:
         periphery, fringe, outer boundary]

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "periphery":
   alentours, ambience, ambit, border, borderlands, bound, boundaries,
   boundary, bounds, bourns, brim, circle, circuit, circumambiencies,
   circumference, circumjacencies, circumscription, circumstances,
   compass, confines, context, coordinates, cortex, covering, crust,
   edge, edges, entourage, envelope, environing circumstances,
   environment, environs, epidermis, exterior, external, facade, face,
   facet, fringe, fringes, front, gestalt, habitat, integument,
   limitations, limits, lineaments, marches, margin, metes,
   metes and bounds, milieu, neighborhood, outer face, outer layer,
   outer side, outer skin, outline, outlines, outposts, outside,
   outskirts, pale, parameters, perimeter, precincts, purlieus, rim,
   rind, shell, situation, skin, skirts, suburbs, superficies,
   superstratum, surface, surroundings, top, total environment, verge,
   verges, vicinage, vicinity