Apocrypha are works, usually written works, that are of unknown authorship, or of doubtful authenticity, or spurious, or not considered to be within a particular canon. The word is properly treated as a plural, but in common usage is often singular. In the context of the Jewish and Christian Bibles, where most texts are of unknown authorship, Apocrypha usually is used by Protestants to refer to a set of texts included in the Septuagint and therefore included in the Catholic canon, but not in the Hebrew Bible.
The word's origin is the Medieval Latin adjective apocryphus, "secret, or non-canonical", from the Greek adjective ἀπόκρυφος (apokryphos), "obscure", from the verb ἀποκρύπτειν (apokryptein), "to hide away".
Apocrypha is commonly applied in Christian religious contexts involving certain disagreements about biblical canonicity. Apocryphal writings are a class of documents rejected by some as being worthy to properly be called Scripture, though, as with other writings, they may sometimes be referenced for support, such as the Book of Jasher. While writings that are now accepted by Christians as Scripture were recognized as being such by various believers early on, the establishment of a largely settled uniform canon was a process of centuries, and what the term "canon" (as well as "apocrypha") precisely meant also saw development. The canonical process took place with believers recognizing writings as being of God, subsequently being followed by official affirmation of what had become largely established. The Roman Catholic church provided its first dogmatic definition of her entire canon in 1546, which put a stop to doubts and disagreements about the status of the Apocrypha, as well as certain other books, which had continued from the beginning of the NT church. The leader of the Protestant Reformation, Martin Luther, like the Catholic church father Jerome (and certain others), favored the Masoretic canon for the Old Testament, excluding apocryphal books in his non-binding canon as being worthy to properly be called Scripture, but included most of them in a separate section, as per Jerome. Luther also doubted the canonicity of four New Testament books (Hebrews, James and Jude, and Revelation), which judgment Protestantism did not follow, but he did not title them Apocrypha.