apsis

Concise Oxford English Dictionary © 2008 Oxford University Press:
apsis /ˈapsɪs/

▶noun (pl. apsides /-diːz/) Astronomy either of the two points in a body's orbit that are nearest to or furthest from the larger body round which it moves.

– derivatives
apsidal adjective.

– origin C17: via L. from Gk apsis, hapsis ‘arch, vault’.