APSIS 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 abiss, by the Latins absis. Apsis, in this sense, amounts to the same with what is otherwise called choir, concha, cannora, and presbyterium; and stands opposite to the navis or body of the church.