The word orthodox comes from two Greek words, ortho + doxa, meaning "right opinion" or "correct thinking."
In Christianity, it generally means adhering to the accepted or traditional historic Christian faith. Some see "orthodoxy" as that which is defined by the early ecumenical creeds which would include the Nicene-Constantinopolitan Creed, popularly known in the West as the Nicene Creed, that was formally accepted by the second Council of Constantinople in 381 A.D. The Apostles' Creed and the Athanasian Creed are accepted as ecumenical in the Western Christian confessions, i.e. the Roman Catholic Church and many Protestant denominations (e.g. see the Lutheran Book of Concord where all three of these creeds are given as "ecumenical").
"Orthodoxy" may also be described as the least common denominator by which an individual, group, or church may legitimately claim the name "Christian." In this sense, orthodox would refer to essential doctrines defining the essence of Christianity.
As a proper name, Orthodox may be used to refer to the Eastern Orthodox churches and/or the Oriental Orthodox churches. It also shows up in some Protestant denominations such as the Orthodox Presbyterian Church (OPC).