Catechism of the Catholic Church §2352 — excerpt
By masturbation is to be understood the deliberate stimulation of the genital organs in order to derive sexual pleasure. "137 "The deliberate use of the sexual faculty, for whatever reason, outside of marriage is essentially contrary to its purpose."
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.