Prose
The word "prose" first appears in English in the 14th century. It is derived from the Old French "prose", which in turn originates in the Latin expression "prosa oratio" (literally, straightforward or direct speech).
Prosimetrum
The term "prosimetrum" is first attested in the "Rationes dictandi" of Hugh of Bologna, in the early 12th century. Sources differ on the date, one suggesting around 1119, another about 1130. Hugh divided metrical composition into three kinds: quantitative verse ("carmina"), verse based on syllable count and assonance ("rithmi"), and "the mixed form ... when a part is expressed in verse and a part in prose" ("prosimetrum"). The derived adjective "prosimetrical" occurs in English as early as Thomas Blount’s "Glossographia" (1656) where it is defined as "consisting partly of Prose, partly of Meteer or Verse".