From the Greek word for "hidden." It normally refers to fifteen books not found in the Hebrew canon of the OT and includes the following: Tobit, Judith, Additions to the Book of Esther, the Wisdom of Solomon, Ecclesiasticus (the Wisdom of Jesus son of Sirach), Baruch, the Letter of Jeremiah, the Prayer of Azariah and the Song of the Three Children, Susanna, Bel and the Dragon, 1 Maccabees, 2 Maccabees, 1 Esdras, the Prayer of Manasseh, and 2 Esdras. Eastern Orthodox churches recognize other books in this category, including 1 Esdras, Psalm 152, the Prayer of Manasseh, 3 Maccabees, and 4 Maccabees. All of these books, with the exception of 2 Esdras, are found in the Septuagint, the Greek version of the OT. However, most of them were almost certainly originally written in Hebrew or Aramaic by pious Jews in the period between c. 250 B.C. and 100 A.D. The Roman Catholic Church has traditionally included the fifteen books in their authoritative canon. These books are called deutero-canonical in the Roman Catholic Church, as distinguished from the thirty-nine proto-canonical books of the Hebrew Bible. Since the Reformation, Protestants have recognized only the proto-canonical books as belonging to the authoritative canon.
Glossary definitions provided courtesy of Church Publishing Incorporated, New York, NY,(All Rights reserved) from "An Episcopal Dictionary of the Church, A User Friendly Reference for Episcopalians," Don S. Armentrout and Robert Boak Slocum, editors.