A metaplasm is a change in the orthography (and hence phonology) of a word. Originally it referred to techniques used in Ancient Greek and Latin poetry, or processes in those languages' grammar.
Many poetic metaplasms are useful for describing processes in the natural development of languages:
Epenthesis, addition of a sound to a word:
beginning of a word (prothesis)
end (paragoge)
Synalepha, two syllables becoming one, occurs by elision, crasis, synaeresis, or synizesis.
Elision ("contraction" in English grammar), removal of a sound:
beginning of a word (apheresis)
middle (syncope)
end (apocope).
Crasis (Ancient Greek contraction), coalescence of two vowels into a new long vowel.
Synaeresis, pronunciation of two vowels as a diphthong. Opposite: diaeresis, pronunciation of a diphthong as two syllabic vowels.
Synizesis, pronunciation of two vowels that do not form a normal diphthong as one syllable, without change in writing. Opposite: hiatus, distinct pronunciation of two adjacent vowels.
Metathesis, rearranging of sounds or features of sounds, may affect vowel lengths (quantitative metathesis).
In rhetoric, metaplasm is the modification of word order for emphasis.
In the grammar of the Romance languages, metaplasm refers to the change in the grammatical gender of nouns from their original gender in Latin.
