Deuterocanonical books (from the Greek meaning 'belonging to the second canon') is a term used since the 16th century in the Catholic Church and Eastern Christianity to describe certain books and passages of the Christian Old Testament that are not part of the Hebrew Bible. The term is used in contrast to the , which are the books contained in the Hebrew canon. The distinction arose as a result of the debate in the early Church as to which texts should be classified as canonical. The term is used as a matter of convenience by the Ethiopian Orthodox Tewahedo Church and other Churches to refer to books of their Old Testament which are not part of the Masoretic Text.
The deuterocanonical books are considered canonical by Catholics, Eastern Orthodox, Oriental Orthodox, and the Church of the East, but are considered non-canonical by most Protestants.
The original usage of the term distinguished these scriptures both from those considered non-canonical and from those considered . However, some editions of the Bible include text from both deuterocanonical and non-canonical scriptures in a single section designated "Apocrypha". This arrangement can lead to conflation between the otherwise distinct terms "deuterocanonical" and "apocryphal".
Philip Schaff says that "the Council of Hippo in 393, and the third (according to another reckoning the sixth) Council of Carthage in 397, under the influence of Augustine, who attended both, fixed the catholic canon of the Holy Scriptures, including the Apocrypha of the Old Testament, ... This decision of the transmarine church however, was subject to ratification; and the concurrence of the Roman see it received when Innocent I and Gelasius I (A.D. 414) repeated the same index of biblical books. Schaff says that this canon remained undisturbed till the sixteenth century, and was sanctioned by the council of Trent at its fourth session," although as the Catholic Encyclopedia reports, "in the Latin Church, all through the Middle Ages we find evidence of hesitation about the character of the deuterocanonicals...Few are found to unequivocally acknowledge their canonicity," but that the countless manuscript copies of the Vulgate produced by these ages, with a slight, probably accidental, exception, uniformly embrace the complete Roman Catholic Old Testament.