Definition of: apsis

(ap′sis) noun plural ap·si·des (ap′sə·dēz)
1. Astron. A point of an eccentric orbit that is nearest to or farthest from the center of attraction, as the aphelion or perihelion of a planet.
2. Geom. The line joining these points to form the major axis of an ellipse.
3. An apse.
4. A reliquary. [< L. See APSE]