A peephole is a small optical device installed in doors where one can see who is on the other side. It is a security device for doors that do not have sidelights to look out of to verify the person at the door is someone you wish to let in.
Some peepholes have a fisheye lens so the viewer can see a wide angle in case there are other unpleasant folks lurking on the porch waiting to burst in for a home invasion. Most hotels have peepholes installed to increase security.
On the exterior, peepholes are obvious and it is easy to block the view. Some folks hang decorations during holidays and accidentally block their peepholes, negating their purpose. Also, some taller folks install peepholes too high for average or shorter folks to utilize. See the peephole optimization node for some tips on how to install them.
Iron Noder 2017