Webster's Dictionary 
(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.
Cyclopedia of Biblical, Theological and Ecclesiastical Literature 
( Ὀρθόξος , From Ὀρθός , Right, and Δόξα , An Opinion ) are those whose doctrine is right-whose religious opinion is in accordance with an assumed or generally prevalent standard. This last is with Roman Catholics the dogmas of their Church, with Protestants it is the Bible. The doctrines which are generally considered as orthodox among us are such as were generally professed at the time of the Reformation, viz. the fall of man, regeneration, atonement, repentance, justification by free grace, etc. The national standard of orthodoxy is not the same in all countries; for those opinions and observances which are received by the majority of any nation, or are patronized by the ruling power, are recognized as the standard faith; hence the Greek Church is orthodox in Russia; the Roman Catholic in Spain, Portugal, France, etc. the Anglican Church in England; the Presbyterian in Scotland; but in Ireland, while the religion of the majority is Roman Catholic, the state Church is on the Anglican model; so that it is a disputed point which set of religious opinions and customs should be acknowledged as orthodox. Again, in Upper Canada the orthodox faith is the Protestant Episcopal; while in Lower Canada the established religion, which is also the opinion of the majority is Roman Catholic. In New England the term is employed to distinguish those Congregational churches which hold the evangelical creed from the Unitarian and Universalist churches. (See Orthodoxy).
Some have thought that, in order to keep error out of the Church, there should be some human form as a standard of orthodoxy, wherein certain disputed doctrines shall be expressed in determinate phrases directly leveled against such errors as shall prevail from time to time, requiring those especially who are to be public teachers in the Church to subscribe or virtually to declare their assent to such formularies. But, as Dr. Doddridge observes,
1. Had this been requisite, it is probable that the Scriptures would have given us some such formularies as these, or some directions as to the manner in which they should be drawn up, proposed, and received.
2. It is impossible that weak and passionate men, who have perhaps been heated in the very controversy thus decided, should express themselves with greater propriety than the apostles did.
3. It is plain, in fact, that this practice has been the cause of great contention in the Christian Church, and such formularies have been the grand engine of dividing it, in proportion to the degree in which they have been multiplied and urged.
4. This is laying a great temptation in the way of such as desire to undertake the office of teachers in the Church, and will be most likely to deter and afflict those who have the greatest tenderness of conscience, and therefore (being equal in other respects) best deserve encouragement.
5. It is not likely to answer the end proposed, viz. the preservation of uniformity of opinion; since persons of little integrity may satisfy their consciences in subscribing what they do not at all believe as articles of peace, or in putting the most unnatural sense on the words. And whereas, in answer to all these inconveniences, his pleaded that such forms are necessary to keep the Church from heresy, and it is better there should be some hypocrites under such forms of orthodoxy than that a freedom of debate and opinion should be allowed to all teachers; the answer is plain that when anyone begins to preach doctrines which appear to those who attend upon him dangerous and subversive of Christianity, it will be time enough to proceed to such animadversion as the nature of his error in their apprehension will require, and his relation to them will admit. These remarks however are not applicable to the use of simple confessions or declarations of faith, the object of which is to ascertain and promote Christian fellowship. The design of these is of course only to state the sense in which we interpret and understand the Word of God. Thus, e.g., the Evangelical Alliance (q.v.) has adopted an orthodox standard for common confession of its members. See Doddridge, Lectures, lect. 174; Watts, Orthodoxy and Charity United; Fuller, Works; Robert Hall, Works; Duncan and Miller, On the Utility of Creeds; Donaldson, Christian Orthodoxy (Lond. 1857, 8vo), especially ch. v. (See Establishment); (See Subscription).