A small hole in a door that lets you see who is outside before you open it, or any small opening that allows you to look through secretly.
From 'peep' (to look quickly, from Middle English 'pepen') combined with 'hole.' The verb 'peep' is imitative, like the sound made by birds. First used for door viewers in the 1800s as a security feature.
Peepholes were a response to the rise of home security concerns in urban areas—they became standard in apartment doors in the late 1800s, and their design (usually with a fish-eye lens) makes the viewer invisible to anyone outside due to refraction.
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