ap·sis 
(ăpsĭs)
(ăpsĭs)n. pl. ap·si·des (-sĭ-dēz′)
1. Architecture An apse.
2. Astronomy The point of greatest or least distance of the orbit of a celestial body from the center of mass of the system.
[Late Latin, from Latin, arch, vault, from Greek hapsis, from haptein, to fasten.]