Spyhole (noun)

A small hole or aperture through which one can observe or spy on someone or something.

Origin:

Late 19th century, from spy + hole.

Examples:

1. There was a spyhole in the door which allowed the prison guard to see into the cell.
2. The spyhole in the wall was used by the spy to observe the enemy's movements.
3. The peephole in the door was so small that it functioned as a spyhole.
4. The spyhole allowed the observer to see the activity in the room without being noticed.
5. The spyhole was covered with a lens to magnify the view for the person observing.