What is 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 called?

squinch — 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. (Webster's 1913.)