‘Conscience’ is a compound word composed of con-, meaning ‘with’, ‘together’, or ‘alongside’ and –science, meaning ‘knowledge’ or ‘knowing’. The New Testament Greek word translated as ‘conscience’ is a very similar compound: syneidēsis (συνείδησις ): the preposition syn– with a form of a verb meaning ‘know’ (a perfect form of ‘see’, eidō, cf. Latin video). Both the Latin and the Greek words suggest that conscience is a kind of knowing that accompanies another knowing. To turn a Latinate word into a more English word, conscience is a ‘co-knowing’.
Both in traditional moral theology and also in popular usage, ‘conscience’ has two broad senses, both of which flow from these roots of the word.
First, conscience may be understood as a person’s general moral sense, his broad perception of right and wrong. Conscience in this sense is ‘little more than our awareness of the distinction between right and wrong’ Even if in concrete, material terms we might differ about what is good and what is evil in a given case, the belief that there is good and evil and that we should strive to do the one and avoid the other is a basic characteristic of sane and decent people. A person who ‘is without conscience’, is a person who seems amoral, who seems to have no sense of right or wrong. A person without conscience does not just behave badly, but also seems not to know or care about the very existence of bad or good. A woman of good conscience is a woman who is morally sensitive, careful to think about the rightness or wrongness of her actions. This very general, very broad sense of conscience does not carry much material content: a conscientious person might often do the wrong thing in fact. Nonetheless, conscience in this general sense lies at the roots of moral seriousness and moral sensitivity, and they in turn are the roots from which good behavior springs.
Secondly, conscience may refer to particular acts of conscience. In this sense conscience is ‘strictly speaking…not a faculty, but an activity, namely, the actual application of moral science to conduct.’ In acts of conscience a person makes particular judgements concerning the rightness or wrongness of moral choices. The ‘con-’ in question is the moral judgement that goes along with or accompanies consideration of a particular course of action under consideration. For instance, when I consider a possible act such as taking a candy bar, I know certain things. I know that the chocolate tastes good, that I am hungry, that the candy bar belongs to a shop owner, and that I have not bought and do not intend to buy the candy bar. In addition to these known facts, I also know or co-know that taking the unpaid for candy is an act of theft and is wrong. The act of judging my contemplated action, of judging it to be an act of theft, is an act of conscience, an exercise of syneidesis.
To give another example, when I consider mowing my aged neighbor’s lawn for her, I consider a number of facts. I think about the act itself and its circumstances: how much time it will take, whether the mower blades are sharp, what will be coolest time of day to mow. In addition to these facts, however, I also consider the goodness or badness of the act under consideration. I think about the act in moral terms and judge it to be something good and kind that should be done.
So ‘conscience’ means both a very general moral sense, which tends to encourage us to act in morally careful and serious ways, but also the particular judgements that accompany contemplated actions and judge those actions as good and desirable or bad and to be avoided. Synderesis is the general moral sense. Syneidesis is my reason making actual moral judgements.
 R.C. Mortimer. The Elements of Moral Theology. London: Adam and Charles Black, 1953. Page 74. Conscience in this sense is called synderesis, a term which probably arose due to a medieval scribe’s misreading of syneidesis. Despite the error, the distinction between synderesis and syneidesis has been maintained in later moral theology to distinguish the two senses for ‘conscience’ discussed in this article.
 Thomas Aquinas. Summa Theologiae, I.lxxix.13.