Such as used to cut the corners of a square tower to support a circular or octagonal dome on top. This is the early, straight-line form of a pendative. It looks to have originally been developed in ancient Persia, and used in Middle Eastern, Byzantine, and especially Islamic architecture. The word is an alteration from around 1500 of earlier scunch or scuncheon, from Middle English sconchon, from Middle French escoinson, esconchon, from es-, out of (from Latin ex-) + coin, angle/wedge (modern quoin). No squinching your eyes shut at this one.
The 12th-century church of San Cataldo, Palermo, has three domes, each supported by four doubled squinches.