A small arch thrown across the corner of a square room to support a superimposed mass, as where an octagonal spire or drum rests upon a square tower; -- called also sconce, and sconcheon.
Arch across an interior angle, e.g. of square tower as support for side of octagon. (Wood, Margaret. The English Medieval House, 415)
A device by which a round dome or drum is supported on a square or polygonal base. The squinch helps transition the weight of the drum or dome to the walls of the square or polygon.