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]