Catechism of the Catholic Church §2505 — excerpt
Truth or truthfulness is the virtue which consists in showing oneself true in deeds and truthful in words, and guarding against duplicity, dissimulation, and hypocrisy.
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.