peephole
nounUK /ˈpiːphəʊl/
US /ˈpiːphoʊl/
A small hole you look through to see who is outside.
She looked through the peephole before opening the door.
OTHER MEANINGS
2. A small opening in a wall or door for looking through.
The old castle wall had a peephole for guards to watch the road.
3. A tiny hole in a machine or container that lets you see inside.
He checked the peephole in the tank to see how much fuel was left.
4. A small hole used for spying or watching secretly.
The child found a peephole in the fence and watched the neighbors' dog.