Conscience
A conscience was an internal moral sense of right and wrong.
In 2364, when Captain Jean-Luc Picard experienced a headache, DaiMon Bok suggested it was his conscience, though it was actually caused by a thought maker he was using against Picard. (TNG: "The Battle")
In 2369, Major Kira Nerys, troubled by the terroristic actions she had taken as member of the Bajoran Resistance, told Kai Opaka that she did not want her to think of her as a violent person without a soul or conscience. (DS9: "Battle Lines")
In 2371, Neelix, questioned Ma'Bor Jetrel's rationale that although he had created the deadly Metreon Cascade weapon, it was government and military leaders who had decided to use it. He asked him if this helped him to sleep at night, to which he replied that he had slept no worse the night before than he had for the past fifteen years. Neelix asked what this meant and he replied that it meant he had to live with his conscience as Neelix did with his, asking Neelix how many he had killed during the war. (VOY: "Jetrel")
In 2374, Captain Benjamin Sisko stated in a personal log he deleted almost immediately afterwards that a guilty conscience was a small price to pay for the safety of the Alpha Quadrant, regarding the morally dubious actions he had taken to bring the Romulan Star Empire into the Dominion War against the Dominion. (DS9: "In the Pale Moonlight")
It was sometimes possible for an individual to act as another's conscience if that person valued their moral opinion. In 2375, when the Doctor faced a conflict between his ethical and cognitive subroutines, Captain Kathryn Janeway chose to erase his memories of what had caused the conflict. When he later discovered the memories on his own, she found herself ready to make the same choice, only for Seven of Nine to object to this course of action. Janeway told her that if she had come to act as her conscience that it was too late, that she had considered the objections that Seven had raised when it had happened before and came to the same conclusion. Seven objected that her conclusion was wrong and when Janeway held fast, told her that she found it unsettling, that she had looked to her as her guide to Humanity, but that perhaps she was mistaken. This caused Janeway to reconsider her decision. (VOY: "Latent Image")