A peephole is a small opening through which one may look.
In a door, a peephole allows people inside the security of seeing outside without opening the door. Glass peepholes are often fitted with a fisheye lens to allow a wider field of view from the inside and little to no visibility from the outside.
I came across this in a book I was reading by Lawrence Block. However, I could find no good explanation for its origin. Anyone have an idea why a peephole is called a Judas Hole? Let me know!