I've always called it the DV Panel...DV for Direct Vision, I was told, but I haven't a clue why "Panel".
There to use for landing if the main windshield becomes opaque for any reason, I was also told, although it always seemed to me unlikely that one could see much ahead through it on most aircraft I've flown.
Mostly used, on light aircraft, for squeaking "Clear Prop", isn't it?