Of foremost importance; paramount: a cardinal rule; cardinal sins. Dark to deep or vivid red. Roman Catholic Church A high church official, ranking just below the pope, who has been appointed by a pope to membership in the College of Cardinals. A dark to deep or vivid red. A North American finch (Cardinalis cardinalis) having a crested head, a short thick bill, and bright red plumage in the male. A short hooded cloak, originally of scarlet cloth, worn by women in the 18th century. A cardinal number.