Catechism of the Catholic Church §1371 — excerpt
The Eucharistic sacrifice is also offered for the faithful departed who "have died in Christ but are not yet wholly purified," so that they may be able to enter into the light and peace of Christ:
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: Council of Trent (1562) DS 1743 · St. Monica, before her death, to her sons, St. Augustine · his brother · Conf. 9, 11, 27: PL 32, 775 · St. Cyril of Jerusalem, Catech. myst. 5, 9. 10 PG 33, 1116-1117