A peephole is a tiny hole you look through, most often in a front door, so you can see who is outside before you open it. Some are just a hole; many doors have a little glass lens, and people still call that a peephole. You also meet the word for a hole in a wall, a fence, or a box when someone wants a quiet look without being seen. It is ordinary household English, a little old-fashioned in speech — some people now say spyhole (British) or door viewer. Though actually, you peep through it: the first half is that small, secret look, not the verb peer, which is a longer, harder stare. Formal writing almost never needs the word; spoken English, safety notices, and crime stories do.