The Collaborative International Dictionary of English v.0.48:

Exterior \Ex*te"ri*or\, a. [L. exterior, compar. of exter or
   exterus on the outside, outward, foreign, strange, a compar.
   fr. ex: cf. F. ext['e]rieur. See Ex-, and cf. Extreme,
   Interior.]
   1. External; outward; pertaining to that which is external;
      -- opposed to interior; as, the exterior part of a
      sphere.
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            Sith nor the exterior nor the inward man
            Resemble that it was.                 --Shak.
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   2. External; on the outside; without the limits of;
      extrinsic; as, an object exterior to a man, opposed to
      what is within, or in his mind.
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            Without exterior help sustained.      --Milton.
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   3. Relating to foreign nations; foreign; as, the exterior
      relations of a state or kingdom.
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   Exterior angle (Geom.), the angle included between any side
      of a triangle or polygon and the prolongation of the
      adjacent side; also, an angle included between a line
      crossing two parallel lines and either of the latter on
      the outside.

   Exterior side (Fort.), the side of the polygon upon which a
      front of fortification is formed. --Wilhelm.
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   Note: See Illust. of Ravelin.
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WordNet (r) 3.0 (2006):

exterior angle
    n 1: the supplement of an interior angle of a polygon [syn:
         exterior angle, external angle]