From Late Latin metathesis, from Ancient Greek μετάθεσις (metáthesis), from μετά (metá, “among”) +‎ θέσις (thésis, “placement”).

metathesis (countable and uncountable, plural metatheses)

the transposition of letters, syllables or sounds within a word

the breaking and reforming of double bonds in olefins in which substituent groups are swapped