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

Fisheye lens

In photography, a fisheye lens is a wide-angle lens that takes in a broad, panoramic and hemispherical image. Originally developed for use in meteorology to study cloud formation and called "whole-sky lenses", fisheye lenses quickly became popular in general photography for their unique, distorted...

 to allow a wider field of view

Field of view

The field of view is the extent of the observable world that is seen at any given moment....

 from the inside and little to no visibility from the outside.

A peephole in any sized wall is a notable feature in the brownstone house of the fictional detective Nero Wolfe

Nero Wolfe

Nero Wolfe is a fictional detective, created in 1934 by the American mystery writer Rex Stout. Wolfe's confidential assistant Archie Goodwin narrates the cases of the detective genius. Stout wrote 33 novels and 39 short stories from 1934 to 1974, with most of them set in New York City. Wolfe's...

. 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

Over My Dead Body (novel)

Over My Dead Body is the seventh Nero Wolfe detective novel by Rex Stout. The story first appeared in abridged form in The American Magazine...

(1939), and undergoes various transformations and improvements thereafter.