noun
peephole; observation window; viewing window
A small window or opening for looking through, typically in a door or wall.
ドアの覗き窓から誰が来たか確認した。
I checked who came through the peephole in the door.
この部屋には外を見るための小さな覗き窓がある。
This room has a small observation window for looking outside.
Similar meaning, but のぞき穴 is a hole rather than a window, often without glass.
Compound of 覗き (のぞき, 'peeping') + 窓 (まど, 'window').