Hagioscope
A hagioscope (from Gr. άγιος, holy, and σκοπός, to see) or squint, in architecture, is an opening through the wall of a church in an oblique direction, to enable the worshippers in the transepts or other parts of the church, from which the altar was not visible, to see the elevation of the host.
Hagioscopes were also sometimes known as "leper windows" wherein a squint was made in an external wall so that lepers and other non-desirables could see the service without coming into contact with the rest of the populace.
In medieval architecture hagioscopes were often a low window in the chancel wall and were frequently protected by either a wooden shutter or iron bars. Hagioscopes are found on one or both sides of the chancel arch; in some cases a series of openings has been cut in the walls in an oblique line to enable a person standing in the porch (as in Bridgwater church, Somerset) to see the altar; in this case and in other instances such openings were sometimes provided for an attendant, who had to ring the Sanctus bell when the Host was elevated.