A small hole in a door or a wall through which you can look, especially without being seen. (Cambridge Dictionary)
A peephole is small hole in a door or wall through which you can look secretly at what is happening on the other side. (Collins Dictionary)
A hole or crevice to peep through. (Merriam – Webster)
A small hole or opening through which to peep or look, as in a door. (Dictionary.com)
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Peephole meaning in the Cambridge Dictionary.