peephole; observation window; viewing window
A small window or opening for looking through, typically in a door or wall.
I checked who came through the peephole in the door.
This room has a small observation window for looking outside.
Standard kanji spelling.
Mixed kanji-kana spelling; also common.
Compound of 覗き (のぞき, 'peeping') + 窓 (まど, 'window').