peephole
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A small opening through a door allowing a person to look from the inside to the outside. Most modern peepholes incorporate a fisheye lens to allow for a wider field of view while also limiting the ability to peer in from the outside.
Not to be confused with peeking.
For when the viewer appears to be looking through it, use pov peephole instead, and only add peephole if one is present in the image itself.