The Collaborative International Dictionary of English v.0.48:

Conscience \Con"science\, n. [F. conscience, fr. L. conscientia,
   fr. consciens, p. pr. of conscire to know, to be conscious;
   con- + scire to know. See Science.]
   1. Knowledge of one's own thoughts or actions; consciousness.
      [Obs.]
      [1913 Webster]

            The sweetest cordial we receive, at last,
            Is conscience of our virtuous actions past.
                                                  --Denham.
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   2. The faculty, power, or inward principle which decides as
      to the character of one's own actions, purposes, and
      affections, warning against and condemning that which is
      wrong, and approving and prompting to that which is right;
      the moral faculty passing judgment on one's self; the
      moral sense.
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            My conscience hath a thousand several tongues,
            And every tongue brings in a several tale,
            And every tale condemns me for a villain. --Shak.
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            As science means knowledge, conscience
            etymologically means self-knowledge . . . But the
            English word implies a moral standard of action in
            the mind as well as a consciousness of our own
            actions. . . . Conscience is the reason, employed
            about questions of right and wrong, and accompanied
            with the sentiments of approbation and condemnation.
                                                  --Whewell.
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   3. The estimate or determination of conscience; conviction or
      right or duty.
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            Conscience supposes the existence of some such
            [i.e., moral] faculty, and properly signifies our
            consciousness of having acted agreeably or contrary
            to its directions.                    --Adam Smith.
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   4. Tenderness of feeling; pity. [Obs.] --Chaucer.
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   Conscience clause, a clause in a general law exempting
      persons whose religious scruples forbid compliance
      therewith, -- as from taking judicial oaths, rendering
      military service, etc.

   Conscience money, stolen or wrongfully acquired money that
      is voluntarily restored to the rightful possessor. Such
      money paid into the United States treasury by unknown
      debtors is called the Conscience fund.

   Court of Conscience, a court established for the recovery
      of small debts, in London and other trading cities and
      districts. [Eng.] --Blackstone.

   In conscience, In all conscience, in deference or
      obedience to conscience or reason; in reason; reasonably.
      "This is enough in conscience." --Howell. "Half a dozen
      fools are, in all conscience, as many as you should
      require." --Swift.

   To make conscience of, To make a matter of conscience, to
      act according to the dictates of conscience concerning
      (any matter), or to scruple to act contrary to its
      dictates.
      [1913 Webster]

WordNet (r) 3.0 (2006):

conscience
    n 1: motivation deriving logically from ethical or moral
         principles that govern a person's thoughts and actions
         [syn: conscience, scruples, moral sense, sense of
         right and wrong]
    2: conformity to one's own sense of right conduct; "a person of
       unflagging conscience"
    3: a feeling of shame when you do something immoral; "he has no
       conscience about his cruelty"

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "conscience":
   anima, censor, coconscious, collective unconscious, compunction,
   conscientiousness, conscious self, death instinct, demur, ego,
   ego ideal, ego-id conflict, ethical self, ethics, fairness,
   foreconscious, grace, honor, id, inner arbiter, inward monitor,
   judgement, libidinal energy, libido, mind, moral censor, morality,
   morals, motive force, persona, personality, pleasure principle,
   preconscious, primitive self, principles, psyche,
   psychic apparatus, racial unconscious, scruple, scruples, self,
   social conscience, standards, subconscious, subconscious mind,
   subliminal, subliminal self, submerged mind, superego,
   tender conscience, twinge of conscience, unconscious,
   unconscious mind, vital impulse, voice of conscience



Easton's 1897 Bible Dictionary:

Conscience
   that faculty of the mind, or inborn sense of right and wrong, by
   which we judge of the moral character of human conduct. It is
   common to all men. Like all our other faculties, it has been
   perverted by the Fall (John 16:2; Acts 26:9; Rom. 2:15). It is
   spoken of as "defiled" (Titus 1:15), and "seared" (1 Tim. 4:2).
   A "conscience void of offence" is to be sought and cultivated
   (Acts 24:16; Rom. 9:1; 2 Cor. 1:12; 1 Tim. 1:5, 19; 1 Pet.
   3:21).


Bouvier's Law Dictionary, Revised 6th Ed (1856):

CONSCIENCE. The moral sense, or that capacity of our mental constitution, by
which we irresistibly feel the difference between right and wrong.
     2. The constitution of the United States wisely provides that "no
religious test shall ever be required." No man, then, or body of men, have a
right to control a man's belief or opinion in religious matters, or to
forbid the most perfect freedom of inquiry in relation to them, by force or
threats, or by any other motives than arguments or persuasion. Vide Story,
Const. Sec. 1841-1843.