יְמֵי הַבֵּינַיִם
The Hebrew word for between is בֵּין
, so that in-between is בֵּינַיִם
.
Medieval times, or the Middle Ages, are thus called יְמֵי הַבֵּינַיִם
– literally, the in-between days.
For example:
הָיוּ וִכּוּחִים פִילוֹסוֹפִיִּם גְּדוֹלִים בֵּין רַבָּנִים בִּימֵי הַבֵּינַיִם.
There were great philosophical arguments among rabbis in Medieval times.
Medieval as an adjective is יְמֵי-בֵּינַיְמִי
, or simply מֶדְיֶוָלִי 