Apsis

n.	1.	(Astron.) One of the two points of an orbit, as of a planet or satellite, which are at the greatest and least distance from the central body, corresponding to the aphelion and perihelion of a planet, or to the apogee and perigee of the moon. The more distant is called the higher apsis; the other, the lower apsis; and the line joining them, the line of apsides.
	2.	(Math.) In a curve referred to polar coördinates, any point for which the radius vector is a maximum or minimum.
	3.	(Arch.) Same as Apse.

Noun

1.

apsis - a domed or vaulted recess or projection on a building especially the east end of a church; usually contains the altar Synonyms: apse

apse, church, church building, niche, recess, tribune