Canon, canons, canonical

From the Greek word kanon (which means ‘measuring stick'), the canon refers to the set list of books understood to belong in the Bible. Therefore a ‘canonical' book is a term often used for a book that is considered to genuinely be part of the Bible. As the Bible developed, books were only believed to be part of the authoritative collection if they ‘measured up' to certain criteria (e.g. they were somehow linked to the apostles, or to Moses).