This is a leper's squint

The leper's squint was a type of hagioscope, a window set at an oblique angle in a church wall to permit people to see the altar from areas where it was not otherwise visible. When the hagioscope went through to the exterior of the church, it was usually intended for use either by lepers or by anchorites (hermits who lived in a cell built against the wall of the church). (Once again, credit to Scribal Terror)