A peephole is a small hole in a door that lets you see who's on the other side without opening it. Most apartment doors and hotel room doors have peepholes installed at eye level, usually with a special lens that gives you a wide view of the hallway outside.

The word combines peep (to look quickly or secretly) with hole (an opening). When you hear a knock, you can walk up to your door and look through the peephole to see if it's your neighbor, a delivery person, or someone you don't recognize. The person outside can't see you looking at them, which makes peepholes useful for safety and security.

Peepholes work because of a fish-eye lens, which bends light to give you a surprisingly wide view through such a tiny opening. Without this lens, you'd only see a tiny circle of whatever was directly in front of the hole.

You might also hear someone use peephole more generally to mean any small opening used for watching or spying, like a hole between rooms or a gap in a fence. In old castles, guards used peepholes in doors and walls to watch for intruders without being seen.