Easton's Bible Dictionary
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; Romans 2:15). It is spoken of as "defiled" (Titus 1:15), and "seared" (1 Timothy 4:2). A "conscience void of offence" is to be sought and cultivated (Acts 24:16; Romans 9:1; 2 Corinthians 1:12; 1 Timothy 1:5, 19; 1 Peter 3:21).
Noah Webster's New International Dictionary of the English Language
1. (n.) Knowledge of the rightness or wrongness of one's own thoughts or actions; consciousness.
2. (n.) 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.
3. (n.) The estimate or determination of conscience; conviction or right or duty.
4. (n.) Tenderness of feeling; pity.