Catechism of the Catholic Church §2483 — excerpt
Lying is the most direct offense against the truth. To lie is to speak or act against the truth in order to lead into error someone who has the right to know the truth.
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.