Orthoepeia means the correct use of words, from the Greek orth- + -epos, correct + word, speech. Older English forms are orthoepy and orthoepia.
The English meaning of orthoepeia is correct pronunciation, or the study of pronunciation. This is the only sense in English acknowledged by the OED and Webster's Dictionary. In this sense, its opposite is barbarism.
However, in ancient Greek, orthoepeia generally had the sense of "correct diction" (cf. LSJ ad loc., or the etymology in the OED); the archaic English term for this subject is orthology, and in this sense its opposite is solecism. The study of orthoepeia by the Greek sophists of the fifth century BC, especially Prodicus (c. 396 BC) and Protagoras, also included proto-logical concepts. Protagoras criticized Homer for making the word for "wrath" feminine (Aristotle, Sophistic Refutations 14) and for praying to the Muse with an imperative (ibid. Poetics 19). Plato depicts Protagoras criticizing the poet Simonides for contradicting himself, and then shows Socrates and Prodicus arguing to the contrary that Protagoras has conflated the senses of the words "be" and "become" (Protagoras 339a-340c). Euripides and Aeschylus bicker over orthotes epeon in Aristophanes' comedy The Frogs.
