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 ἀψίς, 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.

Arsis 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.

Arsis was also used for a reliquary or case, wherein the relics of saints were 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.

Arsis, in Astronomy, a term used indifferently for either of the two points of a planet's orbit, where it is at the greatest or least distance from the sun or earth; and hence the line connecting those points is called the line of the apsides. The word is Greek, and derived from ἀπό, I connect. The apsis, at the greatest distance from the sun, is called the aphelion, and at the greatest distance from the earth the apogee; while that at the least distance from the sun is termed the perihelion, and at the least distance from the earth the perigee.