That's more a linguistic than a historian question, I guess.
Medieval is simply a portmanteau of the Latin words medium ("the middle") and aevum ("age", like "eon").
"medieval" -> "sth. related to the Middle Ages"
The term Middle Ages is the work of Italian historians during the Renaissance, such as Flavio Biondo, who recognized the Middle Ages as a historical period distinctively different from their own during the fifteenth and sixteenth centuries. Historian Conrad Rudolph stated that this was something Biondo’s contemporaries and followers gradually formalized with terms such as media tempestas (1469), media aetas (1518), and media tempora (1531). The actual term medium aevum is first found by at least 1604; with the English equivalent following afterwards.
Source: Conrad Rudolph, A Companion to Medieval Art: Romanesque and Gothic in Northern Europe