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.
      [1913 Webster]

   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
    adj 1: happening or arising or located within some limits or
           especially surface; "internal organs"; "internal
           mechanism of a toy"; "internal party maneuvering" [ant:
           external]
    2: occurring within an institution or community; "intragroup
       squabbling within the corporation" [syn: internal,
       intragroup]
    3: inside the country; "the British Home Office has broader
       responsibilities than the United States Department of the
       Interior"; "the nation's internal politics" [syn: home(a),
       interior(a), internal, national]
    4: located inward; "Beethoven's manuscript looks like a bloody
       record of a tremendous inner battle"- Leonard Bernstein; "she
       thinks she has no soul, no interior life, but the truth is
       that she has no access to it"- David Denby; "an internal
       sense of rightousness"- A.R.Gurney,Jr. [syn: inner,
       interior, internal]
    5: innermost or essential; "the inner logic of Cubism"; "the
       internal contradictions of the theory"; "the intimate
       structure of matter" [syn: inner, internal, intimate]

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "internal":
   bosom, center, central, cerebral, conceptive, conceptual, core,
   deep, deep-seated, deepest recesses, endopsychic, esoteric, gut,
   heart, heart of hearts, home, immanent, implanted, implicit,
   inalienable, indoor, indwelling, infixed, ingrained, inherent,
   inland, inmost, inner, inner landscape, inner life, inner man,
   inner nature, inner recess, inner self, innermost, inside,
   intellectual, intelligent, interior, interior man, intern,
   intestine, intimate, intrados, intrinsic, inward, inwrought,
   irreducible, mental, municipal, national, native, noetic,
   noological, penetralia, phrenic, private, psychic, psychologic,
   rational, reasoning, recesses, resident, secret, secret place,
   secret places, soul, spiritual, subjective, thinking, unalienable,
   unchallengeable, unquestionable, visceral, vital center, vitals