A spyhole is the tiny hole in a front door that lets you look out and see who is standing there before you open it. It usually has a small round lens in it, so the picture is wide but bent. You could also use the word for any small hole made in a wall or a door for secretly watching — in a prison cell door, for instance. This is more of a British word. Americans usually say peephole, and peephole is understood everywhere, so it is the safer one to learn first. Note that the person inside is the one doing the looking; a spyhole protects you rather than exposes you.

門眼、貓眼（台灣一般講「貓眼」或「門上的窺視孔」；美式英文多用 peephole）