mezepoko

Both halves already have pages. Mezo is the middle and epoko is an age or era, so mezepoko is the middle age, and it means what English means by the Middle Ages. Note that Esperanto keeps it singular where English pluralises; la mezepoko is one long stretch of time, not a set of them.

Every European language did the same sum. Medium aevum in Latin, Moyen Age in French, Mittelalter in German, Edad Media in Spanish. Esperanto is not being clever here, it is being ordinary, and that is rather the point: a word built from mezo and epoko needs no glossary entry for a speaker of any of those languages.

Writing it. The adjective is mezepoka, medieval. Period names are often capitalised in Esperanto when they name one specific era, so la Mezepoko and la mezepoko are both current; usage has not settled and neither will get you into trouble. Mezepokisto is a medievalist.

Its neighbours in time. Antikveco is antiquity, Renesanco the Renaissance, jarcento a century, and jarmilo a millennium. The furniture of the period is here too: kastelo a castle, kavaliro a knight, monaĥo a monk.