metathesis
noun
me·tath·e·sis mə-ˈta-thə-səs 
plural metatheses mə-ˈta-thə-ˌsēz
: a change of place or condition: such as
a
: transposition of two phonemes in a word (as in the development of crud from curd or the pronunciation \\ˈpər-tē\\ for pretty)
b
: a chemical reaction in which different kinds of molecules exchange parts to form other kinds of molecules
metathetical
ˌme-tə-ˈthe-ti-kəl
adjective or metathetic
ˌme-tə-ˈthe-tik
metathetically
ˌme-tə-ˈthe-ti-k(ə-)lē
adverb 
Did you know?
One familiar example of metathesis is the English word thrill, which was thyrlian in Old English and thirlen in Middle English. By the late 16th century, native English speakers had switched the placement of the r to form thrill. Another example is the alteration of curd into crud (the earliest sense of which was, unsurprisingly, curd). It probably won't surprise you to learn that the origin of metathesis lies in the idea of transposition—the word was borrowed into English in the mid-16th century and derives via Late Latin from the Greek verb metatithenai, meaning "to transpose."
Word History
Etymology
Late Latin, from Greek, from metatithenai to transpose, from meta- + tithenai to place — more at do
First Known Use
circa 1538, in the meaning defined above
Medical Definition
metathesis
noun
me·tath·e·sis mə-ˈtath-ə-səs
plural metatheses -ˌsēz
: double decomposition