From my dictionary, starting with second definition:
2 5L metaplasmus, an irregularity < Gr metaplasmes, formation of cases of nouns from a missing nom. < meta, over (see META3) + plassein, to form (see PLASTIC)6 a change in a linguistic form made by the addition, omission, or transposition of a sound or sounds or a syllable or syllables

But it is a subset of a group of allied rhetorical terms, the names of which I'll try to relocate.