"semicircular extension at the end of a church," 1846, from L. apsis "an arch, a vault," from Gk. hapsis (Ionic apsis) "loop, arch," originally "a fastening, felloe of a wheel," from haptein "fasten together;" the original sense in Gk. 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 Eng. in the L. form (1706).