0 a hole in the side of building, usually with a cover that can be opened and closed, through which small animals go in and out of the building:
Newborn pigs have access to the box through a pop-hole in the door.
It is not enough to have the odd pop-hole in the side of a shed, even if this ticks a box on a certification scheme.
There are small "popholes" along the walls, so theoretically the birds can go outside.
