A beautiful Latin phrase meaning "happy fault." It refers to the original sin of Adam, which, while a fault, paved the way for greater blessings through Christ's victory over sin.

"Where sin increased, grace abounded all the more" (Romans 5:20).
"There is nothing to prevent human nature's being rasied up to something greater, even after sin; God permits evil in order to draw forth some greater good. Thus St. Paul says, 'Where sin increased, grace abounded all the more'; and the Exultet sings, 'O happy fault, . . . which gained for us so great Redeemer" (St. Thomas Aquinas, Summa Theologiae III, 1, 3, ad 3, cf. Catechism of the Catholic Church par. 412).