The judgment of the practical intellect deciding, from general principles of faith and reason, the goodness or badness of a way of acting that a person now faces.
It is an operation of the intellect and not of the feelings or even of the will. An action is right or wrong because of objective principles to which the mind must subscribe, not because a person subjectively feels that way or because his will wants it that way.
Conscience, therefore, is a specific act of the mind applying its knowledge to a concrete moral situation. What the mind decides in a given case depends on principles already in the mind.
These principles are presupposed as known to the mind, either from the light of natural reason reflecting on the data of creation, or from divine faith responding to God's supernatural revelation. Conscience does not produce these principles; it accepts them. Nor does not conscience pass judgment on the truths of reason and divine faith; it uses them as the premises from which to conclude whether something should be done (or should have been done) because it is good, or should be omitted (or should have been omitted) because it is bad. Its conclusions also apply to situations where the mind decides that something is permissible or preferable but not obligatory.
Always the role of conscience is to decide subjectively on the ethical propriety of a specific action, here and now, for this person, in these circumstances. But always, too, the decision is a mental conclusion derived from objective norms that conscience does not determine on its own, receiving it as given by the Author of nature and divine grace.