peephole
noun [ C ] uk/ˈpiːp.həʊl/ us/ˈpiːp.hoʊl/ (UK also spyhole)
（门或墙上的）窥视孔;猫眼
a small hole in a door or a wall through which you can look, especially without being seen
I have a security peephole in my front door. 我家前门上有个防盗窥视孔。
Synonym: eyehole