Catechism of the Catholic Church §1054 — excerpt
Those who die in God's grace and friendship imperfectly purified, although they are assured of their eternal salvation, undergo a purification after death, so as to achieve the holiness necessary to enter the joy of God.
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.