Catechism of the Catholic Church §1849 — excerpt
Sin is an offense against reason, truth, and right conscience; it is failure in genuine love for God and neighbor caused by a perverse attachment to certain goods.
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: St. Augustine, Contra Faustum 22: PL 42, 418 · St. Thomas Aquinas, STh I-II, 71, 6