in ecclesiastical writers, denotes an inner part in the ancient churches, wherein the clergy sat, and where the altar was placed. It is supposed to have been thus called because covered with an arch or vault of its own, by the Greeks called αψίς, by the Latins absis. Apsis, in this sense, amounts to the same with what is otherwise called choir, concha, camera, and presbyterium; and stands opposed to the nave or body of the church.

Apsis is more particularly used for the bishop's seat in ancient churches. This was peculiarly called apsis gradata, because raised on steps above the ordinary stalls. It was also denominated exedra, and in latter times tribune.

Apsis is also used for a reliquary or case, wherein the relics of saints were anciently kept. It took the name apsis from its being round or arched at the top, or perhaps from the place where it was kept. The apsis was commonly placed on the altar; it was usually of wood, sometimes also of gold and silver, with sculptures, &c.