hagioscopes

Through these hagioscopes -- or "holy spy-holes" -- the prior could see mass being celebrated at the high altar and at the altars below in the transept, without entering the cathedral.

The Cathedral Church of Canterbury [2nd ed.]. Hartley Withers 1908

The exteriors of these are of the same form and size as the crypt windows, but they are deeply splayed inside, and probably were used as hagioscopes or squints, to allow those kneeling in the choir aisles to see the priest celebrating mass at the crypt altar.

Bell's Cathedrals: Wimbourne Minster and Christchurch Priory A Short History of Their Foundation and a Description of Their Buildings Thomas Perkins 1874