Catechism of the Catholic Church §2152 — excerpt
A person commits perjury when he makes a promise under oath with no intention of keeping it, or when after promising on oath he does not keep it.
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.