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.[1]
A peephole in any sized wall is a notable feature in the brownstone house of the fictional detective Nero Wolfe. The peephole in Wolfe's office is covered by a painting that is actually a perforated panel. Anyone wishing to secretly observe what happening in the office can stand in an alcove at the end of the hall, where everything can be seen and heard through a hole positioned at Wolfe's eye level. The peephole makes its first appearance in the novel, Over My Dead Body (1939), and undergoes various transformations and improvements thereafter.[2]