apse: [19] Apse ‘vaulted recess in a church’ is an anglicization of Latin apsis. This was a borrowing of Greek apsís or hapsís, which meant literally ‘a fastening together’ (it was derived from the verb háptein ‘join’). The notion that underlies its application to a vaulted space seems to be the joining together of arcs to form a circle; an early Greek use was as a ‘felloe’, part of the rim of a wheel, and this later came to mean, by extension, the wheel itself.
Further metaphoricization led to the sense ‘orbit’, and, more semicircularly, ‘arch’ or ‘vault’. The Latin/Greek form apsis itself was borrowed into English at the beginning of the 17th century, and remains in use as a technical term in astronomy, ‘extreme point of an orbit’.
"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).
