What is the X in X-mas?
1570s, "transposition of letters in a word;" c.1600, "rhetorical transposition of words," from Late Latin metathesis, from Greek metathesis "change of position, transposition, change of opinion," from stem of metatithenai "to transpose," from meta- "to change" (see meta-) + tithenai "to place, set," from PIE root *dhe- "to put, to do" (see factitious). Plural is metatheses. Related: Metathetic.
metathesis me·tath·e·sis (mĭ-tāth'ĭ-sĭs)
n. pl. me·tath·e·ses (-sēz')
Double decomposition of chemical compounds in which an element or radical of one compound exchanges places with another element or radical in another compound.