A conscience is a built-in sense of what's right and what's wrong. That sick feeling in your stomach after you meanly told your younger brother the truth about Santa Claus? That might be your conscience bothering you.
The word conscience contains the word science, which comes from the Latin word scientia, meaning "to know" or "knowledge." You can think of your conscience as your knowledge of yourself, especially when it comes to your own morals, or your feelings about right and wrong. Pangs of conscience, which feel like an uncomfortable inner voice, are helpful when you're trying to decide the right thing to do in a particular situation.