(1):

(a.) According or congruous with the doctrines of Scripture, the creed of a church, the decree of a council, or the like; as, an orthodox opinion, book, etc.

(2):

(a.) Approved; conventional.

(3):

(a.) Sound in opinion or doctrine, especially in religious doctrine; hence, holding the Christian faith; believing the doctrines taught in the Scriptures; - opposed to heretical and heterodox; as, an orthodox Christian.