Catechism of the Catholic Church §1778 — excerpt
Conscience is a judgment of reason whereby the human person recognizes the moral quality of a concrete act that he is going to perform, is in the process of performing, or has already completed.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: John Henry Cardinal Newman, "Letter to the Duke of Norfolk," V, in Certain Difficulties felt by Anglicans in Catholic Teaching II (London: Longmans Green, 1885), 248