In an article in The Presbyterian Guardian, J. Gresham Machen wrote on the problems of terms commonly used to describe the historic Christian faith. Here are a few excerpts:
Many years ago, …some brilliant person said: “Orthodoxy means ‘my doxy’ and heterodoxy means ‘the other man’s doxy’.”
The unknown author of that famous definition–unknown to me at least–may have thought that he was being very learned. Knowing that the Greek word “heteros,” which forms a part of the English word “heterodoxy,” means “other,” he built his famous definition around that one word, and “heterodoxy” became to him “the other man’s doxy.”
The Machen Seminar 