peephole; spyhole; inspection hole; observation window
A small hole or window for looking through, such as in a door, wall, or equipment.
I checked who was at the door through the peephole.
Look at the internal condition through the inspection hole of the machine.
Standard kanji spelling.
Mixed kana-kanji spelling, also common.
Compound of 覗き (のぞき, 'peeping, looking') + 穴 (あな, 'hole').