Heterocliton, in Grammar, an irregular or anomalous word, which either in declension, conjugation, or regimen, deviates from the ordinary rules of grammar. The word is Greek, ἱστιαῖος; formed of ἱστιαῖος, ἀλλερ, "another, different," and ἰστιαῖος, "I decline."

Heteroclite is more peculiarly applied to nouns which vary or are irregular in point of declension; having fewer cases, numbers, &c. than ordinary; or that are of one declension in one number, and another in another: as Ἱκε τας, τασις; Ἱκε τασις, τασις.