The Collaborative International Dictionary of English v.0.48:

Interior \In*te"ri*or\, a. [L., compar. fr. inter between: cf.
   F. int['e]rieur. See Inter-, and cf. Intimate.]
   [1913 Webster]
   1. Being within any limits, inclosure, or substance; inside;
      internal; inner; -- opposed to exterior, or
      superficial; as, the interior apartments of a house; the
      interior surface of a hollow ball.
      [1913 Webster]

   2. Remote from the limits, frontier, or shore; inland; as,
      the interior parts of a region or country.
      [1913 Webster]

   Interior angle (Geom.), an angle formed between two sides,
      within any rectilinear figure, as a polygon, or between
      two parallel lines by these lines and another intersecting
      them; -- called also internal angle.

   Interior planets (Astron.), those planets within the orbit
      of the earth.

   Interior screw, a screw cut on an interior surface, as in a
      nut; a female screw.

   Syn: Internal; inside; inner; inland; inward.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Internal \In*tern"al\, a. [L. internus; akin to interior. See
   Interior.]
   [1913 Webster]
   1. Inward; interior; being within any limit or surface;
      inclosed; -- opposed to external; as, the internal parts
      of a body, or of the earth.
      [1913 Webster]

   2. Derived from, or dependent on, the thing itself; inherent;
      as, the internal evidence of the divine origin of the
      Scriptures.
      [1913 Webster]

   3. Pertaining to its own affairs or interests; especially,
      (said of a country) domestic, as opposed to foreign; as,
      internal trade; internal troubles or war.
      [1913 Webster]

   4. Pertaining to the inner being or the heart; spiritual.
      [1913 Webster]

            With our Savior, internal purity is everything.
                                                  --Paley.
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   5. Intrinsic; inherent; real. [R.]
      [1913 Webster]

            The internal rectitude of our actions in the sight
            of God.                               --Rogers.
      [1913 Webster]

   6. (Anat.) Lying toward the mesial plane; mesial.
      [1913 Webster]

   Internal angle (Geom.), an interior angle. See under
      Interior.

   Internal gear (Mach.), a gear in which the teeth project
      inward from the rim instead of outward.

   Syn: Inner; interior; inward; inland; inside.
        [1913 Webster] Internal-combustion

WordNet (r) 3.0 (2006):

internal angle
    n 1: the angle inside two adjacent sides of a polygon [syn:
         interior angle, internal angle]