In historical linguistics, metaphony is a class of sound change in which one vowel in a word is influenced by another in a process of assimilation. The sound change is normally "long-distance" in that the vowel triggering the change may be separated from the affected vowel by several consonants, or sometimes even by several syllables.
Progressive (or left-to-right) metaphony, in which a vowel towards the beginning of a word influences a subsequent vowel.
Regressive (or right-to-left) metaphony, in which a vowel towards the end of the word influences a preceding vowel.
Metaphony is closely related to some other linguistic concepts:
Vowel harmony is sometimes used synonymously with metaphony. Usually, however, "vowel harmony" refers specifically to a synchronic process operating in a particular language, normally requiring all vowels in a word to agree in a particular feature (e.g. vowel height or vowel backness). Most commonly, the triggering vowel is in the first syllable of the word (i.e., this is a type of progressive metaphony), as in Turkish, Finnish or Hungarian. In some cases, however, the triggering vowel is in the last syllable, typically a suffix, as in many varieties of Andalusian Spanish.