apse (n.)

"semicircular extension at the end of a church," 1846, from Latin apsis "an arch, a vault," from Greek hapsis (Ionic apsis) "loop, arch," originally "a fastening, felloe of a wheel," from haptein "fasten together," which is of unknown origin. The original sense in Greek seems to have been the joining of the arcs to form a circle, especially in making a wheel. The architectural term is earlier attested in English in the Latin form (1706). Related: Apsidal.